Musnad Ahmad 31:267

Abu Usamah narrated to us, al-Walid ibn Kathir narrated to us, from Muhammad ibn Ka'b, from Ubaydullah ibn Abdullah—and Abu Usamah said once: from Ubaydullah ibn Abd al-Rahman ibn Rafi' ibn Khadij—from Abu Sa'id al-Khudri, who said: It was said: "O Messenger of Allah, shall we perform ablution from Bi'r Bada'ah, and it is a well into which menstrual rags, filth, and the flesh of dogs are thrown?" He said: "The water is pure; nothing makes it impure."