Is it obligatory to change underwear before ablution (wudu') and prayer (salat) due to the discharge of a small amount of fluid from the anus?

The worshipper must purify his clothes from impurities to the best of his ability, as Allah the Almighty says: (And purify your garments). If it is difficult to have a pure garment for prayer, a barrier can be placed to prevent the spread of impurity. If it becomes difficult to do this for every prayer, it is permissible to combine prayers formally to reduce the number of times clothes need to be changed. If all of this is impossible, then there is no harm in praying in the condition one is in due to hardship and because Allah the Almighty says: (Allah does not charge a soul except [with that within] its capacity). A person afflicted with urinary incontinence must perform ablution for each prayer after its time has begun, wash what afflicts his body, and take precautions to prevent the spread of urine.