Musnad Ahmad 26:859

'Affan narrated to us, Mahdi narrated to us, Wasil narrated to us, from Abu Wa'il, who said: We went to 'Abdullah ibn Mas'ud one day after the morning prayer. We greeted him at the door, and he gave us permission to enter. A man from the group said: "I recited the *Mufassal* in its entirety last night." He (Ibn Mas'ud) said: "This is like the reciting of poetry. We have indeed heard the recitation, and I memorize the pairs that the Messenger of Allah, peace and blessings of Allah be upon him, used to recite: eighteen surahs from the *Mufassal*, and two surahs from *Al-Ha-Mim*."