Musnad Ahmad 26:512

Hashim narrated to us, Sayyar informed us, from Abu Wa'il, who said: A man came to Abdullah ibn Mas'ud and said: "I recited the Mufassal in one rak'ah last night." Abdullah said: "Like the scattering of dry dates? And like the reciting of poetry? I indeed know the pairs that the Messenger of Allah (peace and blessings of Allah be upon him) used to join together, two surahs in one rak'ah."