Musannaf Ibn Abi Shayba 1:1629

Hushaym narrated to us, from Abd al-Hamid ibn Ja'far, from Mujammi' ibn Ya'qub ibn Mujammi', that the Messenger of Allah (may Allah bless him and grant him peace) said to 'Uwaym ibn Sa'idah: "What is this purification for which Allah has praised you?" They said: "We wash the posterior."