Abu Waail reported,

A man named Nahik bin Sinan came to Abdullah, with a report like that of Waki, except that he said, Alqama came to go in to him, so we said to him, "Ask him about the comparable surahs that the Messenger of Allah ﷺ used to recite in a single rakat."He went in to him and asked him, then came out to us and said, "Twenty surahs from the Mufassal, according to Abdullah's arrangement."And he said, "I certainly know the comparable surahs that the Messenger of Allah ﷺ used to recite, two in each rakat, twenty surahs in ten rakats."

Muslim 1910

Sahih

و حدثناه إسحق بن إبراهيم أخبرنا عيسى بن يونس حدثنا الأعمش في هذا الإسناد بنحو حديثهما وقال إني لأعرف النظائر التي كان يقرأ بهن رسول الله ﷺ اثنتين في ركعة عشرين سورة في عشر ركعات

مسلم 1910صحيح
Contents
Reference
Commentary
Ayat
Collection
Sahih Muslim
Book
Virtues of the Quran and what relates to it
Chapter
Reciting with measured pace and avoiding hastiness, which is excessive speed, and the permissibility of reciting two or more surahs in a rakat
Numbering
Darussalam 1910
Muslim 1910