Musnad Ahmad 30:3117

Waki' narrated to us, from Hammad, from Thabit, from Abu Rafi', from Abu Hurairah, from the Prophet (peace and blessings of Allah be upon him), who said: "A man went out from his village to visit a brother of his in another village. Allah set an angel in wait for him, and he sat on his path. He said to him: 'Where do you intend to go?' He said: 'I intend to go to a brother of mine whom I am visiting for the sake of Allah in this village.' He said to him: 'Does he have any favor upon you that you are repaying?' He said: 'No, but I have loved him for the sake of Allah, the Almighty and Majestic.' He said: 'Then I am the messenger of your Lord to you: that He has loved you because you loved him for His sake.'"