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Ibn 'Abbas reported: The Messenger of Allah (ﷺ) went to the privy and when he came back, he was presented with food. It was said to him; Messenger of Allah, wouldn't you perform ablution. He said: Why, am I to say prayer?
Source: sunnah.com
Other translations
Arabic
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا مُحَمَّدُ بْنُ مُسْلِمٍ الطَّائِفِيُّ، عَنْ عَمْرِو بْنِ دِينَارٍ، عَنْ سَعِيدِ بْنِ الْحُوَيْرِثِ، مَوْلَى آلِ السَّائِبِ أَنَّهُ سَمِعَ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، قَالَ ذَهَبَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِلَى الْغَائِطِ فَلَمَّا جَاءَ قُدِّمَ لَهُ طَعَامٌ فَقِيلَ يَا رَسُولَ اللَّهِ أَلاَ تَوَضَّأُ . قَالَ " لِمَ أَلِلصَّلاَةِ " .
sunnah.com