Musannaf Ibn Abi Shaybah

Hazrat Ikrimah bin Khalid Makhzumi narrates that a man appointed his slave as a mukatab (contracted for freedom) to serve two slaves; both of them practiced the same profession. Both of them brought their case before Hazrat Umar (may Allah be pleased with him). He gave this verdict: If he does not bring you such slaves who know that profession, then make him a slave again.