It was narrated that 'Abdullah bin Arqam said:
"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).'" (IT)
English reference (IT): Vol. 1, Book 1, Hadith 616 (IT)
Arabic reference (IT): Book 1, Hadith 659 (IT)