Musannaf Ibn Abi Shayba 13:2246

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Ja’far ibn Burqan narrated to us, from Maymun ibn Mihran, who said: Ibn Umar had a writing contract (kitabah) with a slave of his. When the time for his installment came, he (the slave) came with it. He (Ibn Umar) said: "Where did you get this from?" He said: "I was begging and working." He said: "Do you want to feed me the filth of the people? You are free, and you have this installment of yours."