It is directly proved by authentic hadith of the Prophet (peace and blessings be upon him). Though the Qur’an does not directly mention it, Muslims must believe in it without doubt since it is based on the authentic Sunnah of the Prophet (peace and blessings be upon him).
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: While one does not find any explicit mention of punishment in the grave in the Qur’an, such punishments are definitely mentioned in the authentic Sunnah of the Prophet (peace and blessings be upon him) which we must accept as valid because rejecting these will be akin to rejecting the Qur’an itself. The Prophet (peace and blessings be upon him) has been entrusted not only with the duty of conveying the Qur’an to us, but also with the task of explaining its teachings. Therefore, we must accept such narrations of the Prophet (peace and blessings upon him) as being part and parcel of Islamic teachings that must be adhered to without questioning. Having said this, I must add that matters of punishment in the grave or what happens after death, including Heaven and Hell, are all beyond the limits of human cognition and, therefore, we must not speculate about them based on our limited human reason. To use reason to judge or evaluate such matters is no different from using a scale meant for measuring gold to weigh a mountain.
The Prophet (peace and blessings be upon him) has warned us about being careless and neglectful of cleanliness after defecation and urination. Our negligence in this area will entail serious consequences.
now coming to the issue of urinating while standing, it is considered by some scholars as makruh or undesirable simply because of the fact that doing so may cause urine to splash onto one’s body or clothes. We should, therefore, avoid it unless we are faced with a situation where we have no other choice, for instance, while using a public washroom, if there is concern about exposing ourselves to contagious diseases if we were to sit on the toilet.
Makruh refers to something that is considered undesirable. Technically speaking, it refers to an action that we should avoid, and if we avoid it we will be rewarded for doing so, while, at the same time, there is no punishment if we do it. Haram, on the other hand, is that which is prohibited and we must avoid it, for if we did it, we would incur sin and punishment. If we happen to pass urine while standing because of certain unavoidable circumstances, we must make sure that the urine does not splash onto our body or clothes. If it does, we must wash ourselves before we are eligible to perform salah (ritual Prayer).