Musannaf Ibn Abi Shayba 3:214

Abu Bakr narrated to us, he said: Mu’adh ibn Mu’adh narrated to us, from Habib ibn al-Shahid, who said: I heard Muhammad ibn Bashir saying: "The bowing (ruku') is like this." And Mu’adh described that he makes his back level, he does not lower his head, nor does he raise it. He said: I heard al-Hasan saying the same thing, except that al-Hasan spoke about it with some words.