Musnad Ahmad 907:135

Abdullah narrated to us, Surayj ibn Yunus narrated to us, Umar ibn Ubayd narrated to us, from Simak ibn Harb, from Jabir ibn Samurah, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "There will be twelve commanders after me." Then he spoke, but it was unclear to me. So I asked the one who was next to me, or beside me, and he said: "All of them are from Quraysh."