Salaam. My mother-in-law is a Parsi (Zoroastian) from India. My wife converted to Islam and we're both practicing Muslims. We've both made it clear to my in-laws that we don't want to be a part of anything religious from their side. They have understood that and accepted it. Sometimes, on happy occasions, they put a red mark on my wife's forhead and my forehead. The mark is placed in the center of the forehead, half an inch above the center of the eyebrows. They say it's cultural and has no religious significance. I've done some research and also found that it's a cultural thing rather than religious. Can you please tell me, with evidence/sources if that is true and if it is okay?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

First of all, we ask Allaah to reward you for being steadfast on the truth and seeking to abide by the rulings of His religion.

This sign is peculiar to those non-Muslim people and it is not permissible for you to put it on your foreheads as those people do even if this has nothing to do with their creed because it is not permissible for a Muslim to imitate the non-Muslims in matters that are peculiar to them. It is confirmed that the Prophet said: “Whoever imitates a people, he is one of them.” [Ahmad and Abu Daawood]

Allaah Knows best.