Ubayy b. Ka'b said:

The Prophet (ﷺ) was present at the pool of Banu Ghifar, Gabriel came to him and said: "Allah has commanded you to make your community read (the Qur'an) in one harf. He (the Prophet) said: 'I beg Allah His pardon and forgiveness; my community has not strength to do so'. He then came for the second time and told him the same thing till he reached up to seven harfs. Finally, he said: 'Allah has commanded you to make your community read (the Qur'an) in seven harfs; in whichever mode they read, that will be correct.

حَدَّثَنَا ابْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنِ الْحَكَمِ، عَنْ مُجَاهِدٍ، عَنِ ابْنِ أَبِي لَيْلَى، عَنْ أُبَىِّ بْنِ كَعْبٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ عِنْدَ أَضَاةِ بَنِي غِفَارٍ فَأَتَاهُ جِبْرِيلُ صلى الله عليه وسلم فَقَالَ إِنَّ اللَّهَ عَزَّ وَجَلَّ يَأْمُرُكَ أَنْ تُقْرِئَ أُمَّتَكَ عَلَى حَرْفٍ ‏.‏ قَالَ ‏

‏ أَسْأَلُ اللَّهَ مُعَافَاتَهُ وَمَغْفِرَتَهُ إِنَّ أُمَّتِي لاَ تُطِيقُ ذَلِكَ ‏

‏ ‏.‏ ثُمَّ أَتَاهُ ثَانِيَةً فَذَكَرَ نَحْوَ هَذَا حَتَّى بَلَغَ سَبْعَةَ أَحْرُفٍ قَالَ إِنَّ اللَّهَ يَأْمُرُكَ أَنْ تُقْرِئَ أُمَّتَكَ عَلَى سَبْعَةِ أَحْرُفٍ فَأَيُّمَا حَرْفٍ قَرَءُوا عَلَيْهِ فَقَدْ أَصَابُوا ‏.‏