It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both); he said: "The Messenger of Allah (peace and blessings be upon him) had himself cupped and gave him his wage; had it been impure (earnings), he (peace and blessings be upon him) would not have given him the wage."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12792