Musannaf Ibn Abi Shayba 37:342

Muhammad ibn al-Hasan al-Asadi narrated to us, from Ibrahim ibn Tahman, from Sulaym ibn Qays al-Amiri, from Suhaym ibn Nawfal, who said: Abdullah ibn Mas’ud said to me: "How will you be when the prayer-performers fight one another?" I said: "And will that happen?" He said: "Yes, the companions of Muhammad." I said: "And what should I do?" He said: "Restrain your tongue, conceal your place, and hold fast to what you know, and do not abandon what you know for what you deny."