Prayer is one of the basic pillars of Islam. It is the cornerstone of Islam. That is why a Muslim is required to perform it perfectly. When `Umar ibn Al-Khattab (may Allah be pleased with him) summoned his governors for advice and instructions, he said, “The most important thing you are supposed to take care of is the performance of salah. If someone observes it properly he will observe any other religious duty lower than it; and if one neglects it he will neglect everything else.”
Thus, the prayer of the one who knowingly performs it incorrectly is invalid.
Dr. Muzammil Siddiqi, former President of the Islamic Society of North America, states: “Salah should be performed in its prescribed manner at its appointed times. There are some minor differences in the schools of jurisprudence concerning the way salah is to be performed, but all those differences are based on different Prophetic hadiths and the careful interpretations of trustworthy jurists. We can follow any of those trustworthy jurists, but we are not allowed to introduce any new method of salah or to perform it as we wish.
For example, while in the standing position, you can hold your hands on your chest or navel, or you can leave them down at your sides, but you are not allowed to hold them behind your back or raise them on your head. It is haram (forbidden) to deliberately change the form of prayer. The prayer of anyone who knowingly changes the form of prayer and does not follow the Sunnah of the Prophet is not valid, and, in fact, such a person will incur the sin and anger of Allah.”