Ali bin Abd al-A'la علي بن عبد الأعلى · The Book on Hajj

"When Allah revealed: And Hajj to the House is a duty that mankind owes to Allah, for whomever is able to bear the journey. They said: 'O Messenger of Allah! Is that every year?' He remained silent. So they said: 'O Messenger of Allah! Is that every year?' He said: 'No. If I had said yes, then it would have been made obligatory.' So Allah revealed: O you who believe! Do not ask about things which, if made plain to…