Musnad Ahmad 32:647

Husayn and Khalaf ibn al-Walid narrated to us, they said: al-Mubarak narrated to us, he said: Thabit al-Bunani narrated to me, Anas ibn Malik informed me, that a man said to the Prophet, peace and blessings of Allah be upon him: "I love such-and-such a person for the sake of Allah." He said: "Have you informed him?" He said: "No." He said: "Then inform him." So he said to him: "Know that I love you for the sake of Allah." He said: So he said to him: "May the One for whom you loved me love you." And Khalaf said in his narration: "So he met him."