Abu Dawud Book 36, Hadith Number 5106.

Narated By Anas ibn Malik : A man was with the Prophet (PBUH) and a man passed by him and said: Apostle of Allah! I love this man. The Apostle of Allah (PBUH) then asked: Have you informed him? He replied: No. He said: Inform him. He then went to him and said: I love you for Allah’s sake. He replied: May He for Whose sake you love me love you!