Musnad Ahmad 26:418

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Abu Ishaq, from al-Aswad ibn Yazid and Alqamah, from Abdullah, that a man came to him and said: "I recited the Mufassal in one rak'ah." He said: "Rather, you recited it rapidly like the reciting of poetry, or like the scattering of dry dates. But the Messenger of Allah (peace and blessings of Allah be upon him) did not do as you did. He used to recite the pairs: Ar-Rahman and An-Najm in one rak'ah." He said: Abu Ishaq mentioned ten rak'ahs, with twenty surahs according to the arrangement of Abdullah, the last of them being: "When the sun is wrapped up" (At-Takwir) and "The Smoke" (Ad-Dukhan).