Ibn Ulayyah narrates it from al-Jurayri with the same chain. (b) Al-Jurayri says: We asked when sleep is established, and he said: When the sleeper lays his side on the ground. (c) This narration is not established as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 585