Sahih Muslim Book 4, Hadith Number 1709.

Chapter : What has been related (from the Holy Prophet) about one who sleeps the whole night till morning.

Zaid b. Thabit reported that the Apostle of Allah (may peace be upon him) 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.”