Ubayy b. Ka'b reported that the Messenger of Allah (ﷺ) was near the tank of Banu Ghifar that Gabriel came to him and said:

Allah has commanded you to recite to your people the Qur'an in one dialect. Upon this he said: I ask from Allah pardon and forgiveness. My people are not capable of doing it. He then came for the second time and said: Allah has commanded you that you should recite the Qur'an to your people in two dialects. Upon this he (the Holy prophet) again said: I seek pardon and forgiveness from Allah, my people would not be able to do so. He (Gabriel) came for the third time and said: Allah has commanded you to recite the Qur'an to your people in three dialects. Upon this he said: I ask pardon and forgiveness from Allah. My people would not be able to do it. He then came to him for the fourth time and said: Allah has commanded you to recite the Qur'an to your people in seven dialects, and in whichever dialect they would recite, they would be right.
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا غُنْدَرٌ، عَنْ شُعْبَةَ، ح وَحَدَّثَنَاهُ ابْنُ الْمُثَنَّى، وَابْنُ بَشَّارٍ قَالَ 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 - قَالَ - فَأَتَاهُ جِبْرِيلُ عَلَيْهِ السَّلاَمُ فَقَالَ إِنَّ اللَّهَ يَأْمُرُكَ أَنْ تَقْرَأَ أُمَّتُكَ الْقُرْآنَ عَلَى حَرْفٍ ‏.‏ فَقَالَ ‏"‏ أَسْأَلُ اللَّهَ مُعَافَاتَهُ وَمَغْفِرَتَهُ وَإِنَّ أُمَّتِي لاَ تُطِيقُ ذَلِكَ ‏"‏ ‏.‏ ثُمَّ أَتَاهُ الثَّانِيَةَ فَقَالَ إِنَّ اللَّهَ يَأْمُرُكَ أَنْ تَقْرَأَ أُمَّتُكَ الْقُرْآنَ عَلَى حَرْفَيْنِ فَقَالَ ‏"‏ أَسْأَلُ اللَّهَ مُعَافَاتَهُ وَمَغْفِرَتَهُ وَإِنَّ أُمَّتِي لاَ تُطِيقُ ذَلِكَ ‏"‏ ‏.‏ ثُمَّ جَاءَهُ الثَّالِثَةَ فَقَالَ إِنَّ اللَّهَ يَأْمُرُكَ أَنْ تَقْرَأَ أُمَّتُكَ الْقُرْآنَ عَلَى ثَلاَثَةِ أَحْرُفٍ ‏.‏ فَقَالَ ‏"‏ أَسْأَلُ اللَّهَ مُعَافَاتَهُ وَمَغْفِرَتَهُ وَإِنَّ أُمَّتِي لاَ تُطِيقُ ذَلِكَ ‏"‏ ‏.‏ ثُمَّ جَاءَهُ الرَّابِعَةَ فَقَالَ إِنَّ اللَّهَ يَأْمُرُكَ أَنْ تَقْرَأَ أُمَّتُكَ الْقُرْآنَ عَلَى سَبْعَةِ أَحْرُفٍ فَأَيُّمَا حَرْفٍ قَرَءُوا عَلَيْهِ فَقَدْ أَصَابُوا ‏.‏
Reference : Sahih Muslim 821 a
In-book reference : Book 6, Hadith 334
USC-MSA web (English) reference : Book 4, Hadith 1789
  (deprecated numbering scheme)