It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to sleep lying on his back, to the extent that he would begin to snore, then he (peace and blessings be upon him) would wake up and lead the prayer without performing fresh ablution.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4051
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لضعف الحجاج، وله شاهد من حديث ابن عباس عند البخاري: 138، ومسلم: 763.