It is narrated from Sayyiduna Abdullah bin Arqam (may Allah be pleased with him) that he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When the prayer has begun and one of you feels the need for the toilet, let him first go to the toilet."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14060