Yahya related to me from Malik from Hisham ibn Urwa from hisfather that Abdullah ibn al-Arqam used to lead his companions inprayer. The time for prayer came one day and he went to relievehimself. When he returned, he said, "I heard the Messenger of Allah,may Allah bless him and grant him peace, say, 'If you wish todefecate, you should do so before the prayer.' "
USC-MSA web (English) reference: Book 9, Hadith 52
Arabic reference: Book 9, Hadith 383