A prominent Islamic leader and at least 17 of his followers were killed by extremists on Monday for having music in their religious ceremonies in Somalia's northern city of Galkayo, say police.Two suicide bombers exploded in front of the preacher's residence, which is also a Sufi shrine, and then four gunmen stormed the building and opened fire, said Ahmed Awale, a police officer in Galkayo. At least 20 others were injured, he said.Somali security forces responded and there was a battle with heavy gunfire in the building. Three attackers were killed in the shootout and one was captured alive, said Awale.Somalia's extremist rebels, al-Shabaab, have claimed responsibility for the attack.The attack killed Sheikh Abdiweli Ali Elmi, who has hundreds of followers and has been controversial because videos show him and his followers chanting religious poems with music, which some extremists say is not Islamic. Sheikh Elmi had defended his actions, saying music is not contrary to Islamic teachings.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.