Musannaf Ibn Abi Shayba 28:191

Ghundar narrated to us, from Shu'bah, from al-Hakam, who said: I heard 'Amr ibn Maymun say: 'Uthman ibn Hunayf entered upon 'Umar, and I heard him say: "If I were to increase upon every head two dirhams, and upon every jarib of land one dirham and a qafiz of food, it would not harm them nor would it burden them," or a word similar to it. He said: "Yes." He said: So it was forty-eight upon every head, and he made it fifty.