Bowing in Islamic culture is almost entirely reserved for prayer. Muslims are instructed not to bow to any human regardless of authority -- even Muhammad would not allow people to bow to him. Bowing in reverence to another human is considered "shirk," which is a very serious sin in Islam.
Posture When Bowing to Allah
In Muslim etiquette, bowing is reserved for God. Islamic prayer consists of two types of bowing: ruku and sujud. Ruku is done during the salat -- the obligatory daily prayers -- and is done in a standing position with the back parallel to the ground. Sujud, also most commonly associated with the salat, is a full prostration. The person taking this posture places his forehead, nose, palms, knees and feet on the ground.
Bowing to Humans is Shirk
A fatwa, or non-binding religious ruling, by the late but esteemed Saudi cleric Abdullah Ibn Jibreen states plainly that bowing to a human, in most circumstances, is "shirk" -- a grave sin in Islam that means, in this case, to worship someone other than Allah. Allah alone is considered worthy of the type of reverence bowing indicates.
Muhammad Forbids Bowing to Him
Muhammad made it very clear that Muslims should bow to no person -- and that includes him. Book 11, number 2135 of the Abu Dawud collection of Muhammad's sayings tells of a man who wanted to bow to Muhammad. In his travels, the man had seen people bowing to civil authorities, and he thought it was only natural that Muslims should bow to the Messenger of God. Muhammad said, "If you were to pass my grave, would you prostrate yourself before it?" The statement was meant to emphasize Muhammad's finite nature as a human being, and he goes on to state that if wives are not to bow to husbands, humans should not bow to earthly authorities, either.
Bowing is acceptable as long as it is not done as a sign of respect, reverence or worship. Shaykh Abd al-Azeez ibn Baaz, a leading 20th Century Islamic scholar, issued a fatwa stating that bowing, as a posture, is fine "if it is not done as a sign of veneration, rather a person leans forward to greet another if he is short or paralyzed or is sitting down, so he leans forward to greet him."
Guidelines for Muslims in Martial Arts
Martial arts instruction includes bowing to one another and bowing to the teacher. Based on the example of Muhammad, this is categorically forbidden for Muslims. A group of Muslim clerics, according to the Islam Question and Answer website, issued a fatwa on this exact subject, noting that any form of bowing "is a kind of worship, and worship is only for Allah alone."
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