Musannaf Ibn Abi Shayba 1:758

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mughirah, from Ibrahim, who said: If the place in which one performs ghusl (ritual bath) from janabah (major ritual impurity) is one where the water collects, then he should wash his feet when he finishes. And if it is clean, then he need not wash them if he wishes.