Sikhism was a religion primarily associated with India. Its adherents were called Sikhs.
The taxi drivers who drove the Seventh Doctor and Ace from Heathrow Airport to Smithwood Manor were Sikhs. (PROSE: Cat's Cradle: Warhead)
Donna Noble's cousin Janice was married to a Sikh. (PROSE: Ghosts of India)
Ajeeth, servant of Lady Cornelia Basildon-Stone, was a Sikh. (COMIC: The Crystal Throne)