Musnad Ahmad 30:2828

'Abd al-Rahman narrated to us, he said: Hammad narrated to us, from Thabit, from Abu Rafi', from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "A man visited a brother of his in another village. Allah set an angel in wait for him on his path. When he came to him, he said: 'Where are you going?' He said: 'I am visiting a brother of mine for the sake of Allah in such-and-such village.' He said: 'Does he have any favor upon you that you are seeking to repay?' He said: 'No, but I have loved him for the sake of Allah.' He said: 'Indeed, I am a messenger of Allah to you, [informing you] that Allah has loved you just as you have loved him for His sake.'"