Abu Dawud Book 36, Hadith Number 5212.

Narated By A man : Ghalib said: When we were sitting at al-Hasan’s door, a man came along. He said: My father told me on the authority of my grandfather, saying: My father sent me to the Apostle of Allah (PBUH) and said: Go to him and give him a greeting. So I went to him and said: My father sends you a greeting. He said: Upon you and upon your father be peace.