Musnad Ahmad 3943

It was narrated from 'Abdullah that A man came to him and said: I recited al-Mufassal in one rak'ah. He said: Rather you recited it quickly like poetry or like dry dates falling from a branch when it is shaken. But the Messenger of Allah (ﷺ) did not do what you did. He used to recite pairs (of soorahs), ar-Rahman and an-Najm in one rak'ah And Abu Ishaq mentioned ten rak'ahs, with twenty soorahs, according to the order of 'Abdullah, the last (pair) of which was Idhash-shamsu kuwwirat and ad-Dukhan.