Musnad Ahmad 26:408

Hisham ibn Abd al-Malik narrated to us, Abu 'Awanah narrated to us, from Husayn, who said: Ibrahim narrated to me, from Nuhayk ibn Sinan al-Sulami, that he came to Abdullah ibn Mas'ud and said: "I recited the Mufassal tonight in one rak'ah." He said: "Is this like the reciting of poetry, or the scattering of dry dates? It was only divided so that you may divide it. I have indeed learned the 'pairs' that the Messenger of Allah (peace and blessings of Allah be upon him) used to join together: twenty surahs, [starting with] 'al-Rahman' and 'al-Najm'," according to the arrangement of Ibn Mas'ud, "every two surahs in one rak'ah." And he mentioned 'al-Dukhan' and 'Amma Yatasa'alun' in one rak'ah.