Sayyiduna Abdullah bin Arqam (may Allah be pleased with him) set out for Hajj or Umrah, and he was leading the people in prayer. One day, when the iqamah was called for the dawn (Fajr) prayer, he said: "Let one of you step forward and lead the prayer," and he himself went to relieve himself. Then he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When one of you needs to relieve himself and the prayer has begun, let him relieve himself first." This hadith is sahih (authentic) on the condition of Imam Bukhari and Imam Muslim, but the two Shaykhs did not record it.