Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.123.

This is based on Abu Humaid’s description of the Prophet’s prayer: “He would raise his head from his bowing, then stand straight until all of his backbones returned to their places.” (Related by al-Bukhari and Muslim.)

‘Aishah related that when the Prophet raised his head from bowing, he would not prostrate until his back was straight. (Related by al-Bukhari and Muslim.)

Abu Hurairah reported that the Prophet said, “Allah does not look at the prayer of a person who does not straighten his back between his bowings and his prostrations.” (Related by Ahmad. al-Munzhiri considers its chain as good.)