Mishkat al-Masabih
Book 30, Hadith 163
Arabic hadith text
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
Translation
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.