Musnad Ahmad 905
It was narrated that ‘Ali (رضي الله عنه) said:
When this verse `And Hajj (pilgrimage to Makkah) to the house (Ka`bah) is a duty that mankind owes to Allah, those who can afford the expenses (for one`s conveyance, provision and residence)` [Al-`Imran 3:97] was revealed, they said: O Messenger of Allah (ﷺ), is it every year? He remained silent. Then they said: Is it every year? He remained silent, Then they said: Is it every year? He said: `No, if I said yes, it would be obligatory.` Then Allah revealed the words: `O you who believe! Ask not about things which, if made plain to you, may cause you trouble` [Al-Ma’idah 5:101] up to the end of the Αyah.
حَدَّثَنَا مَنْصُورُ بْنُ وَرْدَانَ الْأَسَدِيُّ، حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ الْأَعْلَى، عَنْ أَبِيهِ، عَنْ أَبِي الْبَخْتَرِيِّ، عَنْ عَلِيٍّ، رَضِيَ اللَّهُ عَنْهُ قَالَ لَمَّا نَزَلَتْ هَذِهِ الْآيَةُ ‏{‏وَلِلَّهِ عَلَى النَّاسِ حِجُّ الْبَيْتِ مَنْ اسْتَطَاعَ إِلَيْهِ سَبِيلًا‏}‏ قَالُوا يَا رَسُولَ اللَّهِ أَفِي كُلِّ عَامٍ فَسَكَتَ فَقَالُوا أَفِي كُلِّ عَامٍ فَسَكَتَ قَالَ ثُمَّ قَالُوا أَفِي كُلِّ عَامٍ فَقَالَ لَا وَلَوْ قُلْتُ نَعَمْ لَوَجَبَتْ فَأَنْزَلَ اللَّهُ تَعَالَى ‏{‏يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تَسْأَلُوا عَنْ أَشْيَاءَ إِنْ تُبْدَ لَكُمْ تَسُؤْكُمْ‏}‏ إِلَى آخِرِ الْآيَةِ‏.‏