Musnad Abu Ya'la al-Mawsili 115:359

Ubayd Allah narrated to us, Khalid narrated to us, Sa'id narrated to us, from Qatadah, from Anas, or from people among the companions of the Messenger of Allah (may Allah bless him and grant him peace): That they used to lay down their sides and sleep. Among them were those who would perform ablution, and among them were those who would not perform ablution.