Musannaf Ibn Abi Shayba 3:6260

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mughira informed us, from Ibrahim, that he used to: When he finished the prayer with the taslim, he would turn his face toward us while he was proclaiming the oneness of Allah (tahlil), saying: "There is no god but Allah, alone, with no partner."