He (Urwah) said: "While standing for the prayer he (Abdullah bin Al-Arqam) took a man by the hand leading him forward, he (Abdullah) was in front of the people, and he said: 'I heard Allah's Messenger say: "When standing for the prayer and one of you finds that he has to relieve himself then let him relieve himself first."
Jami` at-Tirmidhi Book 1, Hadith 142