It is narrated from Muhammad bin Amr al-Amiri that he said: I was in a gathering of the Companions of the Messenger of Allah (peace and blessings be upon him), and they were discussing among themselves his prayer (peace and blessings be upon him). Abu Humayd (may Allah be pleased with him) said, and then he mentioned the full hadith, and in it there is also: When he (peace and blessings be upon him) bowed, he would place his hands firmly on his knees and keep space between his fingers, then he would bend his back, neither bending it too far nor exposing his cheeks, that is, not raising (his head) too high.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2550