Musannaf Ibn Abi Shayba 3:6389

Humaid ibn Abd al-Rahman narrated to us, from Sulayman ibn al-Mughirah, who said: A man asked al-Hasan about a man who does not know anything except "Say, He is Allah, [who is] One" (Qul Huwa Allahu Ahad), [asking if he may] lead his people in prayer and repeat it. He said: Yes.