Ja’far ibn Muhammad (AS) known as the Ja’far as-Sadiq (AS) is the sixth Shiite Imam. Imam Sadiq (AS) was born on 17 Rabi al-Avval 83th (702 CE) in Medina. His father, the previous Imam, was named Muhammad al-Baqir (AS) and his mother was called Umi-Farvah. After his father, Imam Sadiq (AS) became the Imam of the Shias and his leadership lasted for about 34 years. The era of Imam Sadiq (AS) was the time of instability and wars, since the Umayyad dynasty were weakened so much due to the battles they had in the Islamic territory. In addition , Abbasid dynasty were trying to topple the Umayyad government and reign over the whole Islamic world. The weakness of the rulers gave Imam Baqir (AS) and Imam Sadiq (AS) a perfect opportunity to spread the teachings of Shiism and train so many Shia scholars. At last, Imam Sadiq (AS) was martyred by the order of the Mansur (an Abbasid Caliph) on 148 H (765 CE). 

What does “Sadiq” mean?

The word “Sadiq” means an honest person who does not tell lies. 
Once, Imam Sajjad (AS), the grandfather of Imam Sadiq (AS) was asked: “Who is the Imam after you?”
“My son Muhammad (AS) who opens knowledge and after him his son Jafar who is called in the skies as ‘Sadiq’” Imam Sajjad (AS) replied. 
He was asked once more: “Why out of you all, he is called “Sadiq” while you all are honest?”
Imam Sajjad (AS) said: “Because the Prophet (PBUH & HP) said: ‘Name my son Jafar ibn Muhammd as “Sadiq” because there will be another Jafar in the future who falsely claims to be an Imam and he is named “liar” by Allah’” (On the contrary, Jafar ibn Muhammad is honest because he is a true Imam). (1)
The liar Jafar mentioned in the hadith is the son of Imam Hadi (AS), the eleventh Imam. 

The knowledge of Imam Sadiq (AS)

All Shiite Imams were known for their abundant knowledge in their time; but among all, Imam Baqir (AS) and Imam Sadiq (AS) were different. Due to the fact that Umayyad dynasty were weak and unstable in those times, Imam Baqir (AS) and Imam Sadiq (AS) had this chance to unveil the abundant knowledge of the Prophet’s household. 
About 4000 people learned from Imam Sadiq (AS) and narrated hadiths from him . (2)
Amr ibn Abi al-Mighdam, a Sunni scholar, says: 
“Every time I looked at the face of Jafar ibn Muhammad (AS), it would make me sure that he is a descendant of the prophets. Once, I saw him in Jamrah (a place in Mecca where the pilgrims gather) and he was telling people: ‘Ask me! Ask me!’” (3)
Hassan ibn Ali al-Vasha, a Shiite hadith narrator, says:
I saw about 900 sheikhs who all would narrate hadiths from Jafar ibn Muhammad (AS) (4).
Once Abu Hanifah, one of the most knowledgeable and popular Imams of the Sunnis, was asked: “Who do you think is the most knowledgeable person in religious fields?” he said: “I do not know anyone more knowledgeable than Jafar ibn Muhammad (AS)” (5).
Imam Sadiq (AS) is mostly known for his knowledge in Islamic laws and because of this, the Shiite school of law is called “Jafari School”. The hadiths narrated from Imam Baqir (AS) and Imam Sadiq (AS) are more than all the hadiths narrated from the Prophet (PBUH & HP) and the other Imams all combined together.

The moral attributes of Imam Sadiq (AS)

In addition to his wisdom and knowledge, Imam Sadiq (AS) was also known for his significant moral qualities. 
Muhammad ibn Talha says: 
“Imam Sadiq (AS) was the greatest person among the Prophet’s household and he was so knowledgeable and he would pray and recite the Quran so much and he was an ascetic person.” (6)
Malik ibn Anass, one of the Sunni school’s head, says: 
Every time I met him, he was in one of these three moods: either praying, fasting or whispering prayers (7).
Hisham ibn Salim, a famous apprentice of Imam Sadiq (AS) says:
Abu-Abdillah (Imam Sadiq (AS)) would always take a bag full of bread, meat and money in the darkness of the nights and go to the house of the needy ones in Medina and give it to them while they didn’t know him. Then, when he died, the people weren’t given those goods anymore and from that, they found out about his identity. (8)

Resources 

1.  Al-Khara’ij, ar-Ravandi, vol.1, pg.268
2.   Al-Irshad, Sheikh Mufid, vol.2, pg.179
3.    Siar A’lam an-Nubalah, Zahabi, vol.6, pg.257
4.    Rijal al-Kashi, vol.1, pg.138-139
5.   Siar A’lam an-Nubalah, Zahabi, vol.6, pg.257
6.    Kash al-Qummah, Irbeli, vol.2, pg.691
7.  Bihar al-Anvar, Majlesi, vol.47, pg.16
8.   Kafi, Koleini, vol.4, pg.8
    

Imagine one of your ordinary days in which you go out in the morning to go to work or the university or anywhere else. You attend some gatherings. You see men and women around you interacting and greeting on different occasions and in various manners, more frequently shaking hands.

You are a member of this community too, and you also used to greet different people with whom you had interactions every day and perhaps it was not important for you if the person you shake hands with is a man or a woman. But now, as a Muslim, you must know that shaking hands in Islam with the opposite gender is forbidden. If this has raised a question in your mind, we will be discussing the issue here.

Psychological Views on Shaking Hands

As a scientific fact, everything we do, or we say, or any other kind of action we perform via every part of our body from legs up to the eyes, or even any thought passing into our minds has energy in itself. Once we bring something into existence, such as an utterance or an action, hence its specific kind of energy, it will never go to an end; but it is transformed and transferred to different targets, leaving its influence on them and first on ourselves.

Thus in our interactions, we are producing some kind of energy. So when people from the opposite genders confront each other and shake hands or kiss or hug each other, as a kind of greeting, what kind of energy is being interchanged between them?  

The Main Reason Behind the Prohibition of Shaking Hands in Islam

God has put some kind of desire in human’s nature by which the opposite sexes are attracted to each other. When women and men who are non-Mahrams, greet each other in any way involving touching their bodies or looking at one another in such a way that arises this desire, they will be leaving a negative effect on each other; although it may feel pleasant.

Once this instinct is provoked, to satisfy their sensual needs, people may go further in their relationships and may be driven into immoral and sinful acts. The final result would be regret, distrust among people and broken families and other mental and physical consequences. Because in Islam’s viewpoint, sensual desires must only be satisfied among spouses. Due to this and some other reasons we might not know, God has set some rules regarding the quality of relationships between women and men. Following these rules will make concepts like marriage and family meaningful.

We Can Control Ourselves

One might say that we do not have any bad intentions in our relationships with those who are considered non-Mahram for us. We can control our desires when confronting them and nothing sinful happens. But the prophet Mohammad (PBUH) and his family who were the purest ones of all time obeyed these rules too.

They avoided any kind of interaction with non-Mahrams that are considered to be Haram. 
When people came to the prophet for Bay’ah (to swear allegiance), he shook hands with men, and for women, a container of water was brought in which the prophet put his hand and took his hand out and then women put their hands in the water to make their pledge [2]. 

The Islamic Etiquette of Touching, Looking and Talking

A Muslim is only allowed to touch the body of those of the same gender and those of the opposite gender who are Mahram for him/her.
A man can only look at the face and hands of a non-mahram woman if they do not have decorations and on the condition that it is not for pleasure [i]. 

In the Quran, we read: “Say to the believers, that they cast down their eyes and guard their private parts… And say to the believing women, that they cast down their eyes and guard their private parts, and reveal not their adornment, save such as is outward…” (24:30-31). Non-Mahrams must be careful with their speaking, too. In another part of Quran, we read: “…be not abject in your speech, so that he in whose heart is sickness may be lustful, but speak honorable words.” (33:32). In these verses of the Holy Quran, the danger of arousing sexual desires through speaking and looking has been warned. However, there are no limits to touching, looking, and talking between spouses.

And some exceptions may occur. For example, if a doctor of the same gender is not available, one can go to a doctor from the opposite gender and touching and looking is permitted in this case. But only in case of necessity and as much as needed.

How to Avoid Shaking Hands in Islam with Non-mahrams

Most probably it will be hard for you to find an excuse to refuse to shake hands with non-mahrams especially those with whom you had usual interactions before. Because in your society this may result in a misunderstanding about your attitude toward people.

When a non-mahram wants to shake a hand with you, if you politely say that due to religious matters you can't shake hands but you are pleased to meet them, in most cases, they will accept it from you without being offended. This would be better rather than falsely mentioning illness and other things. But if you explained your reason honestly and someone reacted badly, you do not need to bother yourself convincing that person.

To conclude, shaking hands, kissing, hugging, and every other kind of greeting and interaction between non-Mahrams which involves touching and also looking at each other on purpose, and talking with each other in a tempting manner are Haram in Islam.

Notes:
[i] Refer to the article “The Islamic etiquette of looking” for more information.                     

References: 

1. shakin hands in islam
2. Mohammad ibn Jarir Tabari, “Tarikh-e Tabari,” vol.3, p.61-62.