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.