Musannaf Ibn Abi Shayba 35:281

Hammad ibn Khalid narrated to us, from Malik, from Abdullah ibn Abi Bakr, who said: I heard Aban ibn Uthman and Hisham ibn Isma'il teaching the warranty (uhdah) for slaves: [it is] three days for fever and abdominal pain, and a warranty of one year for insanity and leprosy. And he mentioned that Abu Hanifah said: If they have separated, he does not have the right to return [the slave] except for a defect that was present in him.