One of the worst forms of theft or cheating is cheating in prayer.

The Messenger of Allah (peace and blessings of Allah be upon him) said: The worst type of thief is the one who steals from his prayer.
The people asked, O Messenger of Allah, how can a person steal from his prayer?
He said: By not doing rukoo and sujood properly.
(Reported by Imaam Ahmad, 5/310; see also Saheeh al-Jaami, 997).

This lack of composure and failure to pause in rukoo and sujood and to stand up straight after rukoo or sit up properly between sujoods may be observed in many of those who pray, and hardly any mosque is free of examples of people who do not have the proper composure in prayer. Correct composure is one of the pillars of prayer, without which prayer is invalid.

This is a serious matter.

The Prophet (peace and blessings of Allah be upon him) said: A man’s prayer is not good enough until his back is straight in rukoo and sujood.
(Reported by Abu Dawud, 1/533; see also Saheeh al-Jaamia 7224).

Abu Abdullaah al-Ashari reported that the Prophet (peace and blessings of Allaah be upon him) led his Companions in prayer, then he sat with a group of them. A man came in and started to pray, but made his movements rapid like a chicken pecking the ground.

The Prophet (peace and blessings of Allah be upon him) said: Do you see this? Whoever dies having done this has died outside of the community of Muhammad, and his prayer is like a crow pecking blood. The person who bows then pecks in his sujood is like a hungry man who eats no more than one or two dates what good will that do him?
(Reported by Ibn Khuzaymah in his Saheeh 1/332; see also al-Albaani, Sifat Salaat al-Nabi (The Prophet’s Prayer described), 131).

Zayd ibn Wahb said: Hudhayfah saw a man who was not performing rukoo and sujood properly.
He said: You have not prayed, and if you were to die, you would die on a way other than that revealed by Allah to Muhammad (peace and blessings of Allaah be upon him).
(Reported by al-Bukhaari, see al-Fath, 2/274).

Once a person is aware of this ruling, if he fails to perform prayer with the proper composure, he should repeat it and repent to Allah for what is past; he does not need to repeat all of his previous prayers, as is indicated by the hadeeth Repeat your prayer, for you have not prayed.