مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 342
Purification
How to act when relieving oneself - Section 1
وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَدْخُلُ الْخَلَاءَ فَأَحْمِلُ أَنَا وَغُلَامٌ إِدَاوَةً مِنْ مَاءٍ وَعَنَزَةً يستنجي بِالْمَاءِ
Anas said, "When God’s messenger entered the privy, a servant and I used to carry a skin of water and a pointed staff, and he would cleanse himself with the water.” (Bukhari and Muslim.)
Mishkat al-Masabih, 342مُتَّفَقٌ عَلَيْهِ (الألباني)