Allah's Messenger said: “You will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you of something which, if you do it, you will love one another? Spread the greeting of salam among yourselves.”
Sahih Muslim, no. (54), Sunan Abi Dawud, no. (5193), Sunan at-Tirmidhi, no. (2688), Sunan Ibn Majah, no. (68).
The Companion of Allah's Messenger, Ammar bin Yasir, may Allah be pleased with him, said: “Three things, whoever gathers them has gathered faith: ☆ giving justice from oneself. ☆ greeting people with salam freely. ☆ spending (in the way of Allah) despite being in hardship.”
Sahih Mawquf, Musannaf Ibn Abi Shaybah, ed. ash-Shathri, no. (32461), Shu'ab al-Iman of al-Bayhaqi, ar-Rushd print, no. (10726), Tahdhib al-Athar of at-Tabari, no. (194), al-Bukhari mentioned it mu'allaq before hadith (28), and al-Albani graded it Sahih in “Takhrij al-Kalim at-Tayyib” no. (297).
In al-Bayhaqi's chain, Abu Ishaq as-Sabi'i stated clearly that he heard it; also, in at-Tabari's chain the student of Abu Ishaq is Shu'bah, who narrates from Abu Ishaq and his other teachers only what they stated clearly they heard.
Abdullah bin Amr, may Allah be pleased with them both, narrated: “A man asked the Prophet : Which deed in Islam is best? He said: You should feed people, and greet with salam those you know and those you do not know (everyone).”
Sahih al-Bukhari, no. (12), Sahih Muslim, no. (39), Sunan Abi Dawud, no. (5194), Sunan an-Nasa'i, no. (5000), Sunan Ibn Majah, no. (3253).