Abu Dawud Book 2, Hadith Number 1470.

Chapter : The quran was sent down in seven modes of reading.

Narated By ‘Umar b. al-Khattab : I heard Hisham b. Hakim (b. hazim) reciting Sura al-Furqan in a different manner from my reciting, and the Apostle of Allah (PBUH) had taught me to recite it. I nearly spoke sharply to him, but I delayed till he finished. Then I caught his cloak at the neck, and I brought him to the Apostle of Allah (PBUH). I said: Apostle of Allah, I heard this man reciting Surah al-Furqan in a manner different from that in which you taught me to recite it. The Apostle of Allah (PBUH) then told him to recite. He then recited it in the manner I heard him recite. The Apostle of Allah (PBUH) said: Thus was it sent down. He then said to me: Recite. I recited (it). He then said: Thus was it sent down. He said: The Qur’an was sent down in seven modes of reading, so recite according to what comes most easily.