It was narrated from Thawban that:
The Messenger of Allah said: "No one among the Muslims should stand to pray when he is suppressing (the need to urinate or defecate), until he has to relieve himself." (IT)
English reference (IT): Vol. 1, Book 1, Hadith 619 (IT)
Arabic reference (IT): Book 1, Hadith 662 (IT)