Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 163

وَعَنْهُ قَالَ: إِنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ دَخَلَ الْخَلَاءَ فَوَضَعْتُ لَهُ وَضُوءًا فَلَمَّا خَرَجَ قَالَ: «مَنْ وَضَعَ هَذَا؟» فَأُخْبِرَ فَقَالَ: «اللَّهُمَّ فقهه فِي الدّين» . مُتَّفق عَلَيْهِ

He said: The Prophet entered the privy and I placed water for him. On coming out he asked who had placed it, and when he was told he said, “0 God, give him insight regarding the religion.” Bukhari and Muslim.