Reference: Fataawa al-‘Aqeedah – Question 479, Page 463
Question: We joined one of the Karate clubs in America and the trainer said [to us]: “It is required of you to take an initial bow when you are bowed to” [at the start of the karate dual], but we refused and explained to him about our religion and he accepted. We were, however, told that we were only required to bow with our heads (i.e. drop our head down towards our opponent and then back up again) because he had greeted us with a bow and it was therefore imperative that we return the greeting. So what is your opinion regarding that?
Response: It is not permissible to greet a Muslim or [even] a disbeliever by bowing; Not with any top part of the body and nor with the head, because bowing is a greeting of worship, and worship is for Allaah alone.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.