Sunan ad-Darimi

Sayyiduna Abu Mas’ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: “A man’s prayer is not correct until he keeps his back straight in bowing and prostration.”

Sayyiduna Abu Qatadah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "The worst of people in terms of theft is the one who steals from his prayer." It was asked: O Messenger of Allah! How can someone steal from his prayer? He said: "By not performing the bowing (ruku') and prostration (sujud) properly (correctly)."

It is narrated from Sayyiduna Rifa’ah and Malik (may Allah be pleased with them), who were both brothers, sons of Rafi’, and among the people of Badr. They said: We were sitting with the Messenger of Allah (peace and blessings be upon him), or he said: The Messenger of Allah (peace and blessings be upon him) was sitting and we were around him (this doubt was from Hammam), when suddenly a man entered and, facing the Qiblah, began to pray. When he finished the prayer, he came and greeted the Messenger of Allah (peace and blessings be upon him) and those present. The Messenger of Allah (peace and blessings be upon him) replied to him with “Wa ‘alaik” and said: “Go and pray again, your prayer has not been performed.” So the man went back and began to pray. We were observing his prayer attentively, but we could not discern what was lacking in his prayer. Then, when he finished the prayer, he came and greeted the Messenger of Allah (peace and blessings be upon him) and the others. The Messenger of Allah (peace and blessings be upon him) replied with “Wa ‘alaik” and said: “Go and pray again, your prayer has not been performed.” Hammam said: I do not know whether you told him to repeat the prayer two times or three times. Then the man said: I have not neglected anything in performing the prayer correctly; I do not know what defect or shortcoming you have observed in my prayer. So the Messenger of Allah (peace and blessings be upon him) said: “The prayer of any one of you is not complete until he performs ablution properly, as Allah Almighty has commanded him: so he should wash his face and his hands up to the elbows, then wipe his head, and wash his feet up to the ankles. Then he should say the takbir, praise Allah Almighty, then recite as much of the Qur’an as he is able (i.e., as much as Allah, the Mighty and Majestic, has permitted in this regard), then say the takbir and bow (ruku’), placing his hands on his knees until all his joints are at rest and relaxed. (When rising from ruku’) he should say «سمع الله لمن حمده» and stand up straight, making his back straight and every joint (bone) return to its place. Then, saying Allahu Akbar, he should prostrate and firmly place his face on the ground.” Hammam said: And sometimes he said: “Place his forehead on the ground until all his joints are at rest and relaxed, then say Allahu Akbar and sit properly and straighten his back.” In this way, he described the method for four rak’ahs. When he had explained, he said: “The prayer of any one of you will not be complete until he does so.”