It was narrated that 'Abdullah bin Mughaffal said:
"The Messenger of Allah said: 'None of you should urinate in his wash area for most of the insinuating thoughts come from that.'" (Da'if)Abu 'Abdullah bin Majah said: ("Abul-Hasan said: 'I heard Muhammed bin Yazid saying:) ''Ali bin Muhammed At-Tanafisi said: 'This (prohibition) applies to cases where the ground (in the place used for washing) was soft. But nowadays this does not apply, because the baths you use now are built of plaster, Saruj and tar; so if a person urinates there then pours water over it, that clears it away, and that is fine.'"
English reference: Vol. 1, Book 1, Hadith 304
Arabic reference: Book 1, Hadith 323