Musannaf Ibn Abi Shayba 3:6391

Yahya ibn Sa'id narrated to us, from Sulayman ibn al-Mughirah, that Abu al-Mundhir asked al-Hasan and said: "The prayer is established, and I lead my people in prayer, and I do not recite anything except 'Say, He is Allah, [who is] One' (Qul huwa Allahu ahad), repeating it." He said: "Yes."