Musnad Ahmad 32:490

Zayd ibn al-Hubab narrated to us, Husayn ibn Waqid narrated to us, Thabit al-Bunani narrated to me, Anas ibn Malik narrated to me, he said: I was sitting with the Messenger of Allah, peace and blessings of Allah be upon him, when a man passed by. A man from the people said: "O Messenger of Allah, I truly love this man." He said: "Have you informed him of that?" He said: "No." He said: "Stand up and inform him." He said: So he stood up to him and said: "O you, by Allah, I truly love you for the sake of Allah." He said: "May the One for whom you have loved me love you."