So many of the followers of Sikhism were both Hindu and Muslim. A lot of the terminology of Sikhism borrows words and borrows ideas from both Islam and Hinduism. The word Allah is used to refer to God in parts of the Guru Granth Sahib, but so is the word Ram. One of the first and closest followers of Guru Nanak, who was with him most of his life, was Bhai Mardana. And even today, many pictures of Guru Nanak would have Bhai Mardana in them. Guru Nanak famously, as someone who was spiritually precocious and spiritually curious, traveled through India and Persia and the Middle East, so he visited many temples, but there's also many historic accounts of him going to Mecca and performing hajj. The Guru Granth Sahib, which is considered the spiritual book, the 11th and final Guru for the Sikhs, of the contributors to them, you have Kabir, who grew up in a Muslim family.