Musnad Abu Ya'la al-Mawsili 117:168

A man said, "O Messenger of Allah, I love such-and-such a person for the sake of Allah." He said, "Have you informed him?" He said, "No." He said, "Go to him and inform him." So he went to him and informed him, and he said, "O so-and-so, I love you for the sake of Allah." He said, "May the One for whom you loved me, love you."