Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) says: I recited a verse, and Sayyiduna Ibn Mas'ud (may Allah be pleased with him) recited it in a different way. We came to the Prophet (peace and blessings be upon him), and I said: Did you not teach me such-and-such a verse in this way? He (peace and blessings be upon him) said: "Of course!" Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: Did you not teach me such-and-such a verse in this way? He (peace and blessings be upon him) said: "Of course!" Then he (peace and blessings be upon him) said: "Each of you is reciting well and excellently." I said: How can both of us be?
The narrator says: He (peace and blessings be upon him) struck my chest with his hand and said: "O Ubayy, I was taught the Qur'an, then I was asked: In one mode or in two modes? The angel who was with me said: In two modes. I said: In two modes. Then I was asked: In two modes or in three? The angel with me said to me: Say three! I said: In three modes; until it reached seven modes. Then he said: Every mode in it is 'shafin kafin' (healing and sufficient). If I said: 'Ghafurun Rahim' (Forgiving, Merciful), 'Alimun Halim' (Knowing, Forbearing), 'Sami'un 'Alim' (Hearing, Knowing), 'Azizun Hakim' (Mighty, Wise) and others like them (saying so makes no difference); however, it is not correct to replace a verse of punishment with a verse of mercy, or a verse of mercy with a verse of punishment."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3989