Musnad Ahmad 26:604

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, that he heard Abu Wa'il narrating that a man came to Ibn Mas'ud and said: "I recited the entire Mufassal in one rak'ah." 'Abdullah said: "Is this like the reciting of poetry? I have indeed learned the pairs that the Messenger of Allah, peace and blessings of Allah be upon him, used to join together." He said: "Then he mentioned twenty surahs from the Mufassal, two surahs in every rak'ah."