He slept in the bed they were watching

On the night of the Hijrah, the Prophet's ﷺ house was being watched. The plan required whoever was outside to keep believing he was still inside.

Ali, a young man at the time, lay down in the Prophet's ﷺ bed and covered himself, so the house would look occupied while the Prophet ﷺ and Abu Bakr got clear of the city.

There is a second half to this that is easy to overlook. The Prophet ﷺ had been called al-Amin, the trustworthy, long before the revelation, and people in Makkah — including people who were now his opponents — had left their valuables with him for safekeeping. Those deposits were still in the house.

So Ali did not leave with the others. He stayed behind in a hostile city to return every one of those deposits to its owner, and only then set out for Madinah.

It is worth sitting with that. The property belonged to people who were at that moment trying to hunt the Prophet ﷺ down. It was given back anyway.