Abu Dawud Book 21, Hadith Number 3844.

Narated By Jabir ibn Abdullah : AbulHaytham ibn at-Tayhan prepared food for the Apostle of Allah (PBUH), and he invited the Prophet (PBUH) and his Companions. When they finished (food), the said: If some people enter the house of a man, his food is eaten and his drink is drunk, and they supplicate (to Allah) for him, this is his reward.