Sahih Muslim 1826
صحيح مسلم#1826SahihZaid b. Thabit reported that the Messenger of Allah (ﷺ) made an apartment in the mosque of mats, and he observed in it prayers for many nights till people began to gather around him, and the rest of the hadith is the same but with this addition:" Had this (Nafl) prayer become obligatory for you, you would not be able to observe it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 6:254