It was narrated that 'Aishah said:"The Prophet went out to urinate, and 'Umar followed him with water. He said: 'What is this, O 'Umar?' He said: 'Water.' He said: 'I have not been commanded to perform ablution every time I urinate. If I did that it would have become a Sunnah.'"
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ عَبْدِ اللَّهِ بْنِ يَحْيَى التَّوْأَمِ، عَنِ ابْنِ أَبِي مُلَيْكَةَ، عَنْ أُمِّهِ، عَنْ عَائِشَةَ، قَالَتِ انْطَلَقَ النَّبِيُّ ـ صلى الله عليه وسلم ـ يَبُولُ فَاتَّبَعَهُ عُمَرُ بِمَاءٍ فَقَالَ " مَا هَذَا يَا عُمَرُ " . قَالَ مَاءٌ . قَالَ " مَا أُمِرْتُ كُلَّمَا بُلْتُ أَنْ أَتَوَضَّأَ وَلَوْ فَعَلْتُ لَكَانَتْ سُنَّةً " .
IbnMaja
The Book of Purification and its Sunnah • Hadith 327