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 the cupper his wage; had it been unlawful earnings, the Messenger of Allah (peace and blessings be upon him) would never have given him this wage."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12794