Musannaf Ibn Abi Shayba 3:3923

Ghundar narrated to us, from Ibn Jurayj, from ‘Ata’, regarding [the verse]: "They used to sleep but little of the night." He said: That was when they were commanded to perform the night prayer. Abu Dharr would tie his waist-wrapper and take a staff, leaning upon it. They remained like that until the concession was revealed: "So recite what is easy of it."