Abu Dawud Book 2, Hadith Number 1472.

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

Narated By Ubayy b. K’ab : The Prophet (PBUH) said: Ubayy, I was asked to recite the Qur’an. I was asked: In one mode or two modes? The angel that accompanied me said: Say in two modes, I said: In two modes, I was asked: In two modes or three? So I said: In three modes. The matter reached up to seven modes. He then said: Each mode is sufficiently health-giving, whether you utter “all hearing and all-knowing” or instead “all-powerful and all-wise.” This is valid until you finish the verse indicating punishment on mercy and finish the verse indicating mercy or punishment.