The Prophet ﷺ urinated and Umar was standing behind him with a jug of water. He said: What is this, Umar? He replied: Water for you to perform ablution with. He said: I have not been commanded to perform ablution every time I urinate. If I were to do so, it would become a sunnah.  

بَالَ رَسُولُ اللَّهِ ﷺ فَقَامَ عُمَرُ خَلْفَهُ بِكُوزٍ مِنْ مَاءٍ فَقَالَ مَا هَذَا يَا عُمَرُ فَقَالَ هَذَا مَاءٌ تَتَوَضَّأُ بِهِ قَالَ مَا أُمِرْتُ كُلَّمَا بُلْتُ أَنْ أَتَوَضَّأَ وَلَوْ فَعَلْتُ لَكَانَتْ سُنَّةً