He has a string with a thousand knot, that he would not sleep until he makes tasbeeh using it!

Mas’air said: The beasts in the sea would calm in the sea, while Prophet Joseph, Peace Be upon Him, would not stop the remembrance of Almighty Allah.

Abu Hurirah used to have a string with a thousand knot which he would not sleep until he makes tasbeeh using it, i.e. mentioning Allah’s name for a thousand times.