What is the required manner of straightening the back during bowing (Ruku)?

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One's back must be straightened so that it is level, neither sloping forward nor holding the head excessively high (Qan')—rather, it should be in a middle position. The Prophet, peace be upon him, would straighten his back upon bowing until if a cup of water were placed on his back, it would not move, indicating the perfect levelness of his back. Furthermore, it is recommended to keep the arms away from the flanks (Mujafah), as narrated by Abu Humayd, who mentioned the Prophet, peace be upon him, placing his hands on his knees as if grasping them, and then stretching his hands away from his sides.