Musnad Ahmad 658:81

Ibn Numayr narrated to us, Malik informed us, from Abu Dawud, who said: I met Al-Bara’ ibn ‘Azib, and he greeted me, took my hand, and smiled in my face. He said: "Do you know why I did this to you?" I said: "I do not know, but I do not see you having done it except for good." He said: "The Messenger of Allah (peace and blessings of Allah be upon him) met me and did to me the same as I have done to you. He asked me, and I said the same as you said to me." He then said: "There are no two Muslims who meet, and one of them greets his companion and takes his hand—not taking it except for the sake of Allah the Almighty—but they do not part until they are both forgiven."