Musannaf Ibn Abi Shayba 3:2679

Hammad ibn Khalid narrated to us, from Abd al-Rahman ibn Abi al-Mawali, from Umar ibn Abi Muslim, who said: The nephews of Urwah ibn al-Zubayr used to bathe in the bathhouse on Friday. Urwah would say, "O my nephews, you have only bathed in the bathhouse to remove dirt, so bathe for Friday."