Some people have criticised Allama Ahmad Sa'id Chitrori for asserting that the urine of the Prophet (peace be upon him) is not pure. However, all the narrations which imply its purity are in fact weak and cannot be used as a proof. See here for further detail. Hence, the principle is that the Prophet is a Bashar (mortal human being) and resembles ordinary human beings in every way, generally speaking, except where there is an evidence for a specific difference. Allama wrote:
شرع اسلام میں ایسا نہیں پایا جاتا جس میں پیشاب کو پاک کہا گیا ہو خواہ وہ نبی اللہ صلی اللہ علیہ وسلم کا کیوں نہ ہو
Transation: There is nothing in the Shari'ah of Islam to suggest that urine is pure, even that of the Prophet of Allah peace be upon him.
Reference: Qur'an Muqaddas aur Bukhari Muhaddis p. 35