Abu Dawud Book 2, Hadith Number 1473.
Chapter : The quran was sent down in seven modes of reading.
Narated By Ubayy b. K’ab : The Prophet (PBUH) was present at the pool of Banu Ghifar, Gabriel came to see him and said: Allah has commanded you to make your community read (the Qur’an) in one mode. He (the Prophet) said: I beg Allah His pardon and forgiveness; my community has no strength to do so. He then came for the second time and told him the same thing till he reached seven modes. Finally, he said: Allah has commanded you to make your community read (the Qur’an) in seven modes; in whatever mode they read, that will be correct.