Purity: Ruling on Withholding Faeces and Urine
Practical Laws of Islam as per the teaching of Ayatullah Sistani
Ruling 78
It is disapproved to withhold passing faeces and urine. If withholding it is in a general sense harmful for the person, it is unlawful.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)