Bayhaqi 2553 — The description of bowing

It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him); the narrator said: I think he attributed it to the Prophet (peace and blessings be upon him), and Abu Mu'awiyah was in doubt about this: "Ablution is the key to prayer; what makes it forbidden (to do other things) is the takbir (tahrimah), and what makes it permissible is the taslim; in every two rak'ahs there is a taslim; there is no prayer, obligatory or voluntary, without the recitation of Surah al-Fatihah and some other surah along with it; when one of you bows, let him not make a hump like a donkey (that is, in bowing he should not stretch out his back and lower his head below his back), but let him keep his back straight; and when he prostrates, let him also keep his back straight, for a person prostrates on seven bones: on his forehead, on his two hands, on his two knees and on the front parts (toes) of his feet; and when he sits, let him keep his right foot upright and spread his left foot."