It is narrated from Sayyiduna Abdullah bin al-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 been called (iqamah) and one of you needs to relieve himself, let him first relieve himself."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14050