Hazrat Abu Mas'ud narrates that the Noble Prophet (peace and blessings be upon him) said: That prayer is not sufficient in which a person does not keep his back (completely) straight in bowing and prostration.

Hazrat Ali (may Allah be pleased with him) bin Yahya bin Khallad narrates from his father, from his uncle who was a participant of Badr, that we were sitting with the Noble Prophet (peace and blessings be upon him) when a man entered to perform prayer. He offered a light (i.e., quick) prayer. He neither completed the bowing (ruku) nor the prostration (sajdah). The Prophet (peace and blessings be upon him) was watching him, but the man did not realize it. Thus, he prayed in this manner and then came forward and greeted the Prophet (peace and blessings be upon him) with salam. The Prophet (peace and blessings be upon him) replied and said, "Repeat (the prayer), for you have not prayed." The man did this three times. Each time, the Prophet (peace and blessings be upon him) said, "Repeat (the prayer), for you have not prayed."

It is narrated about Hazrat Miswar bin Makhramah that he saw a man who was not completing his bowing (ruku) and prostration (sajdah). So he said to him, "Pray again!" That man refused. So he did not leave him until he repeated it. And (Imam) Abu Hanifa's opinion is mentioned that: this prayer will suffice for him but he has done wrong.