Musnad Ahmad Hadith 21149

It is narrated from Sulaiman bin Surad (may Allah be pleased with him) on the authority of Ubayy bin Ka'b (may Allah be pleased with him) that I recited a verse, and Ibn Mas'ud (may Allah be pleased with him) was reciting it in another way. I presented myself to the service of the Holy Prophet (ﷺ) and said, "O Messenger of Allah, did you not teach me such and such verse in this way?" The Holy Prophet (ﷺ) said, "Why not?" I said, "This person claims that you taught him this same verse in another way?" The Holy Prophet (ﷺ) struck his hand on my chest, by which all those doubts were removed, and after that, I never had such doubts again. Then the Holy Prophet (ﷺ) said, "Jibreel (peace be upon him) and Mika'il (peace be upon him) came to me. Jibreel (peace be upon him) said, 'Recite the Holy Qur'an on one letter.' Mika'il (peace be upon him) said, 'Request an addition from them.' Jibreel (peace be upon him) said, 'Recite it on two letters.' Mika'il (peace be upon him) again said, 'Request an addition from them,' until it reached seven letters. And he said, 'Each of them is sufficient and healing. If you say « غفورًا رحيماً » or « سميعًا عليمًا » or « عليمًا سميعًا », then Allah is like that, provided that you do not change a verse of punishment into mercy or a verse of mercy into punishment.'"