Musannaf Ibn Abi Shayba 21:491

Ibn Fudayl narrated to us, from Yazid, from Mujahid, who said: I traveled with Ibn Umar. When it was the time of the last part of the night (al-sahar), he would call out: "A listener has heard [the praise of] Allah and His blessing and His good trial upon us. O Allah, be our companion and favor us," three times. "O Allah, I seek refuge in You from Hellfire," three times.