Musannaf Ibn Abi Shayba 1:1416

Abu Bakr narrated to us, he said: Hushaym and Ibn ‘Ulayyah narrated to us, from al-Jariri, from Khalid ibn ‘Alaq al-‘Absi, from Abu Hurayrah, who said: "Whoever has earned sleep, then wudu has become obligatory upon him." Ibn ‘Ulayyah added: Al-Jariri said: "So we asked about the 'earning' of sleep, and he said: 'When he places his side [on the ground].'"