Sayyiduna Anas (may Allah be pleased with him) says: The Companions of the Prophet (peace and blessings be upon him) would lay their sides down (that is, sleep lying on their sides); some of them would then perform ablution again, and some would not perform ablution (again).
Al-Bazzar recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1289