Musnad Ahmad 3933

It was narrated from Naheek bin Sinan as-Sulami that he went to 'Abdullah bin Mas'ood and said: Last night I recited al-Mufassal in one rak'ah. He said: This is like reciting poetry quickly, like dry dates falling from a branch when shaken. Rather it was divided so that you might read it with ease. I learned the pairs that the Messenger of Allah (ﷺ) used to put together in one rak'ah, twenty soorahs: ar-Rahman and an-Najm, according to the order of Ibn Mas'ood, each two soorahs in one rak’ah. And he mentioned ad-Dukhan and "Amma yatasa'aloon in one rak’ah.