Sahih Muslim Book 2, Hadith Number 555.

Chapter : It is forbidden to urinate in stagnant water.

Hammam b. Munabbih said: Of the ahadith narrated to us by Abfi Huraira from Muhammad the Messenger of Allah (may peace be upon him) one is this: The Messenger or Allah (may peace be upon him) said: You should not urinate in standing water, that is not flowing, then wash in it.