Musannaf Ibn Abi Shayba 28:854

Waki’ narrated to us, he said: Al-Rabi’ ibn al-Subayh narrated to us, from Qays ibn Sa’d, from Mujahid, who said: I asked Ibn Umar about participating in military expeditions with the tyrannical leaders, even though they have introduced innovations. He said: "Participate in the expeditions."