Sunan Abi Dawud

Narrated Anas ibn Malik: A man was with the Prophet (ﷺ) and a man passed by him and said: Messenger of Allah! I love this man. The Messenger of Allah (ﷺ) 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