Harm of Holding Urine

Imam Reza (a):
مَنْ أَرَادَ أَنْ لَا یَشْتَكِیَ مَثَانَتَهُ فَلَا یَحْبِسِ الْبَوْلَ وَ لَوْ عَلَى ظَهْرِ دَابَّتِه‏؛
“Whoever wants to avoid complaints about their bladder should not hold their urine, even if they are riding on their mount.”
Ṭibb Al-Reza: 35