Musnad Ahmad 834:30

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, Ash'ath informed me, from al-Hasan, from 'Abd Allah ibn Mughaffal, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not one of you urinate in his bathing place, then perform ablution in it, for indeed most of the whispering (waswas) comes from it."