Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.147A.

When one bows, one’s hands must reach one’s knees. It is sunnah to make the height of the head equal to that of the hips. The hands should be supported by the knees and should be apart from one’s sides. The hands should be open upon one’s knees and thighs, and the palms should be flat. It is reported that ‘Uqbah ibn ‘Amr would bow with his arms separated, his hands on his knees, and his fingers opened beyond his knees. He said, “This is how I saw the Messenger of Allah pray.” (Related by Ahmad, Abu Dawud and an-Nasa’i.)

Abu Humaid reported that when the Prophet, upon whom be peace, bowed, he would be straight, his head neither up nor down (with respect to his hips), and he would place his hands on his knees as if he was holding them.” (Related by an-Nasa’i.)

Muslim records ‘Aishah reporting that when the Prophet bowed, his head would be neither risen nor lowered, but rather between those two positions. Said ‘Ali, “If you put a cup of water on the back of the Prophet, upon whom be peace, while he was bowing, its contents would not spill.” This is related by Ahmad. Abu Dawud recorded it in his Kitab al-Muraseel.

Said Mus’ab ibn Sa’d, “I prayed next to my father. I joined both of my hands and put them between my thighs (while bowing). He stopped me and said, ‘We used to do that, but were later ordered (by the Prophet) to put our hands on our knees.”‘ (Related by “the group.”)