Musannaf Ibn Abi Shayba 18:1313

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from 'Asim, from Ibn Sirin, who said: 'Umar ibn al-Khattab used to allow the slave to seek aid against his master. He sought aid. Muhammad said: My father sought aid against Ibn 'Umar.