It was narrated that 'Ali said: "When the following was revealed: "And Hajj (pilgrimage to Makkah) to the House (Ka'bah) is a duty that mankind owes to Allah, for whoever can bear the way." They asked: '0 Messenger of Allah, is Hajj every Year? He remained silent. They asked: 'Is it every year? He said: 'No. I had said yes, It wouId have become obligatory.' Then the following was revealed: "0 you who believe! Ask not about things which, if made plain to you, may cause you trouble. (Hasan)