Musnad Ahmad 32:1591

Mu'ammal narrated to us, Hammad narrated to us, Thabit narrated to us, from Anas, who said: A man passed by the Prophet, may Allah bless him and grant him peace, and there was a man sitting with the Prophet. The man said: "By Allah, O Messenger of Allah, I truly love this person for the sake of Allah." The Messenger of Allah, may Allah bless him and grant him peace, said: "Have you informed him of that?" He said: "No." He said: "Stand up and inform him, so that affection may be established between you." So he stood up to him and informed him, saying: "I love you for the sake of Allah," or he said: "I love you for Allah." The man replied: "May the One for whom you loved me love you."