Musnad Ahmad 30:1570

Ishaq narrated to us, Muhammad ibn Ammar, the mu'adhdhin of the Mosque of the Messenger of Allah (peace and blessings of Allah be upon him), narrated to us, he said: I heard Sa'id al-Maqburi saying: I heard Abu Hurairah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the best of earnings is the earning of the hand of a worker if he is sincere."