Musnad Ahmad 30:3607

Sulayman ibn Dawud narrated to us, Shu'bah informed us, from Abu Balj, he said: I heard 'Amr ibn Maymun narrating from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is pleased to find the taste of faith, let him love a servant, not loving him except for the sake of Allah, the Almighty and Majestic."