Musnad Ahmad 32:1680

'Affan narrated to us, Hammad narrated to us, he said: Humayd informed us, from Anas, who said: When the people of Yemen approached, the Messenger of Allah (peace and blessings of Allah be upon him) said: "The people of Yemen have come to you; they are softer of heart than you." Anas said: And they were the first to bring the practice of handshaking.