Zaid bin Thabit · The Book of Prayer - Travellers

The Messenger of Allah (ﷺ) made an apartment with the help of the leaves of date trees or of mats. The Messenger of Allah (ﷺ) went out to pray in it. People followed him and came to pray with him. Then they again came one night and waited (for him), but the Messenger of Allah (ﷺ) delayed in coming out to them. And when he did not come out, they cried aloud and threw pebbles at the door. The Messenger…

" Had this (Nafl) prayer become obligatory for you, you would not be able to observe it."