It wasnarrated that 'Ali said:
"When the following was revealed:"AndHajj (pilgrimage to Makkah) to the House (Ka'bah) is a duty thatmankind owes to Allah, for whoever can bear the way." [3:97] Theyasked: 'O Messenger of Allah, is Hajj every year?' He remainedsilent.They asked: 'Is it every year?' He said: 'No. If I hadsaid yes, itwould have become obligatory.' Then the following wasrevealed: "O youwho believe! Ask not about things which, if madeplain to you, maycause you trouble.'" [5:101]
English reference: Vol. 4, Book 25, Hadith 2884
Arabic reference: Book 25, Hadith 2995