Companion Bilal bin Rabah was the first muezzin (caller to prayer) of Islam. A former Ethiopian slave, he was brutally tortured in the Meccan desert but stood firm, repeating 'Ahad, Ahad' (One, One). He was bought and freed by Abu Bakr. Caliph Omar famously said of him: 'Abu Bakr is our master, and he freed our master.' His shrine is in Wadi al-Seer, Amman.