Umar used to say, "Abu Bakr is our master, and he freed our master," meaning Bilal.
كان عمر يقول: أبو بكر سيدنا، وأعتق سيدنا يعني بلالا.
Bukhari narrated it in Fadail al-Sahabah (3754), from Abu Naeem, who said, Abdul-Aziz bin Abu Salama narrated to us, from Muhammad bin Munkadir, who said, Jabir bin Abdullah informed us, he said, and he mentioned it.