Musnad Ahmad 923:1

'Abd al-Rahman ibn Mahdi narrated to us, Hammam narrated to us, from Qatadah, from Yahya ibn Ya'mar, from Sulayman ibn Surad, from Ubayy ibn Ka'b, who said: I recited a verse, and Ibn Mas'ud recited it differently. So I went to the Prophet, peace and blessings of Allah be upon him, and said: "Did you not teach me to recite such-and-such a verse?" He said: "Yes." Then Ibn Mas'ud said: "Did you not teach me to recite it such-and-such?" He said: "Yes. Both of you are doing well and are correct." He (Ubayy) said: So I said to him [the Prophet], and he struck my chest, and said: "O Ubayy ibn Ka'b, it was said to me: 'Recite the Quran.' I said: 'Upon two modes (ahruf)?' He said: 'Upon two modes, or three?' The angel who was with me said: 'Upon three.' I said: 'Upon three,' until it reached seven modes. There is not one of them except that it is healing and sufficient. If you say: 'Forgiving, Merciful,' or you say: 'All-Hearing, All-Knowing,' or 'All-Knowing, All-Hearing,' then Allah is like that, as long as you do not seal a verse of punishment with mercy, or a verse of mercy with punishment."