Musannaf Ibn Abi Shaybah

Hazrat Aisha (may Allah be pleased with her) narrates that when the Prophet (peace and blessings be upon him) performed ruku, he would neither lower his head too much nor keep it completely straight, but would keep it between these two positions.

A man from Thaqif says that I asked Hazrat Abu Hurairah about bowing (ruku‘), so he said: In ruku‘, avoid bending your back like a bow and avoid lowering your head and raising your back.

Hazrat Ka'b says that when you perform ruku, keep your face towards the Qibla and place both hands on your knees. And do not lower your head so much that it goes below your back like the head of a donkey.

Hazrat Mughira narrates that Ibrahim disliked it when a person raised his head too high in bowing (ruku) or made it completely straight.

Hazrat Mujahid disliked bending the back like a bow while performing ruku.

Hazrat Habib bin Shaheed says that I heard Hazrat Muhammad bin Sirin saying that this is how the bowing (ruku) is performed. Then Hazrat Mu'adh described its form in this way: a person should keep his back completely straight, such that he neither keeps his head completely straight nor raises it. He says that I have also heard Hasan saying the same, although Hasan used to discuss this matter.

Hazrat Abdur Rahman bin Abi Laila narrates that when the Holy Prophet (peace and blessings be upon him) was in the state of bowing (ruku), his posture was such that if water were poured between his two shoulders, it would remain there and not flow off due to the levelness.