Jami` at-Tirmidhi 3055

Narrated Abu Al-Bukhtari:from 'Ali who said: "When (the following) was revealed: And Hajj to the House is a duty that mankind owes to Allah, for those who are able to undertake the journey (3:97). They said: 'O Messenger of Allah! Every year?' But he was silent. So they said: 'O Messenger of Allah! Every year?' He said: 'No. If I were to say yes, then it would be required.' And Allah, Mighty and Sublime is He, revealed O you who believe! Ask not about things which, if made plain to you, may cause you trouble (5:)