"The Messenger of Allah said: 'If anyone of you needs to defecate and the immediate call to prayer (Iqamah) is given, let him start with (relieving himself).'" (Using translation from Ibn Mājah 616)
"إِذَا أقيمتْ الصلاةُ وَوَجَد أحدُكُم الغائط فليبدأ بالغائِط".