Musnad Ahmad 316:1

Yahya ibn Sa'id narrated to us, from Hisham, who said: My father narrated to me, from Abdullah ibn Arqam, that he departed from Makkah, and he used to lead them in prayer, give the adhan, and perform the iqamah. One day, he performed the iqamah for prayer, then said: "Let a man from among you lead you in prayer, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'If one of you intends to go to the toilet and the prayer has been established, let him go to the toilet.'"