From Sahih Muslim (Hadith 1826):
Zaid 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
Reference: Muslim 1826