Question:

We joined a karate club in America, and when our trainer told us that we had to bow to him after he bowed to us, we refused, explaining to him that we couldnt because of our religion. He accepted our excuse but then he said that we had to bow our heads at least. He said that since he began the greeting by bowing, we had to respond. What is the ruling in this issue?

Answer:
To greet somebody by bowing is unlawful, regardless whether that person is a Muslim or a disbeliever, and regardless whether you bow with our entire upper body or with your head only. This is because bowing is a kind of worship, and worship is for Allaah Almighty alone. May Allaah send peace and blessing upon Muhammad, his family, and his Companions.
Permanent Committee for Research and Verdicts
Fatawa Islamiyah Vol. 8 Page 219