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).'" (RU)
English reference (RU): Vol. 1, Book 1, Hadith 616 (RU)
Arabic reference (RU): Book 1, Hadith 659 (RU)