Qawwali is a devotional form of music, prevalent among the sufis. The lyrics are in praise of Allah, Prophet Mohammad, members of Prophet's family or renowned Sufi saints. It is written in Persian, Urdu and Hindi and is composed in a specific raga. Qawwali is usually sung in a group, with one or two lead singers. Originally it was sung to the beat of the daff. However, now the Qawwali singing is accompanied by the dholak, tabla, manjira and the harmonium. Traditionally, qawwali is performed outside the shrines of Sufi saints on their birth or death anniversaries. Several theories exist for the evolution of Qawwalis in India. According to one, qawwali evolved from qaul, a form of vocal music similar to the tarana. Amir Khusro (1254-1325) is believed to have incorporated meaningful words into the qaul, which over a period of time developed into qawwali. According to another belief, qawwali originated in Persia in the 10th century AD with the emergence of the Chisti order of Sufism. It was brought to India in the 12th century. The Sabri brothers, Aziz Nazaan, Aziz Mian, Late Nusrat Fateh Ali Khan and Late Aziz Warisi are important names in qawwali singing in the Indian sub-continent.