Reference: Sharh ‘Umdatil-Fiqh – Tape No.9, Question No.15
Question: Why have the scholars mentioned that if a small amount of blood falls on clothing, then it is excused, however a small amount of urine is not excused?
Response: That is because there is a difference of opinion [amongst the scholars] regarding the impurity of blood, whilst there is consensus [amongst them] regarding the impurity of urine.