Musnad Ahmad 31:132

Abd al-Samad ibn Abd al-Warith narrated to us, Abd al-Aziz ibn Muslim narrated to us, he said: Mutarrif narrated to us, from Khalid ibn Abi Nawf, from Sulayt ibn Ayyub, from the son of Abu Sa’id al-Khudri, from his father, who said: I arrived at the Prophet, peace and blessings of Allah be upon him, while he was performing ablution from the well of Buda’ah. I said: "O Messenger of Allah, do you perform ablution from it, while there is thrown into it what is thrown of filth?" He said: "Indeed, water is not made impure by anything."