Hisham bin Urwah narrates from his father, who narrates from Sayyiduna Abdullah bin al-Arqam (may Allah be pleased with him), that he said: The Messenger of Allah (peace and blessings be upon him) said: "When the prayer is about to begin and one of you needs to relieve himself, he should first relieve himself." This hadith has an authentic (sahih) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.