Sunan at-Tirmidhi (Jami-al-Tirmidhi), Chapter: Taharah (Purification)

reported from his father who reported from Sayyidina Abdullah ibn Arqam (RA).He (Urwah) said, "The iqamah for the Salah was called when he (Abdullah ibn Arqam) held a man between his hand and pulled him forward while he himself was the imam. He said he heard the Allah's Messenger (SAW) say that if the iqamah is called and one has the urge to relieve oneself then one must go to the latrine first." [Ahmed15959, Abu Dawud 88, Nisai 851, Ibn e Majah 616]