مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4036
Musnad of Abdullah bin Mas'ood
It was narrated that Abu Wa'il said: A man came to 'Abdullah bin Mas'ood and said: Last night l recited al-Mufassal in one rak'ah. 'Abdullah said: Is it like dry dates falling from a branch when shaken, is it a quick recitation like reciting poetry? I learned the pairs that the Messenger of Allah (ﷺ) used to put together, two soorahs in one rak'ah.
Musnad Ahmad 4062Its isnad is saheeh, al-Bukhari (4996) and Muslim (822)]