Musnad Ahmad 1061:4

Ishaq ibn Isa narrated to us, Ismail ibn Ayyash narrated to us, from Yahya ibn Said, from Urwah ibn al-Zubayr, from Abu Humayd al-Sa'idi, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The gifts of workers are ghulul (ill-gotten gains/treachery)."