Musnad Ahmad 239:1

Yahya ibn Sa'id narrated to us, from Hisham ibn 'Urwah, who said: My father informed me, from 'Abdullah ibn Arqam, that he performed Hajj and used to lead his companions in prayer; he would call the Adhan and the Iqamah. One day, he performed the Iqamah for prayer and said: "Let one of you lead the 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.'"