sleep of the just

Make your mind easy, Percival. Sleep, my son, the sleep of the just; and see what I will do for you when daylight comes to help us both.

Towards the close of the day (the 8th) the holy father [Pope Leo XII], who had never lost his recollection, fell into a profound lethargy, from which he did not recover till at nine in the morning of the 10th [of February, 1829], to fall into the sleep of the just.