Narrated Abu Huraira:

I followed the Prophet () while he was going out to answer the call of nature. He used not to look this way or that. So, when I approached near him he said to me, "Fetch for me some stones for ' cleaning the privates parts (or said something similar), and do not bring a bone or a piece of dung." So I brought the stones in the corner of my garment and placed them by his side and I then went away from him. When he finished (from answering the call of nature) he used, them .

حَدَّثَنَا أَحْمَدُ بْنُ مُحَمَّدٍ الْمَكِّيُّ، قَالَ حَدَّثَنَا عَمْرُو بْنُ يَحْيَى بْنِ سَعِيدِ بْنِ عَمْرٍو الْمَكِّيُّ، عَنْ جَدِّهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ اتَّبَعْتُ النَّبِيَّ صلى الله عليه وسلم وَخَرَجَ لِحَاجَتِهِ، فَكَانَ لاَ يَلْتَفِتُ فَدَنَوْتُ مِنْهُ فَقَالَ ‏ "‏ ابْغِنِي أَحْجَارًا أَسْتَنْفِضْ بِهَا ـ أَوْ نَحْوَهُ ـ وَلاَ تَأْتِنِي بِعَظْمٍ وَلاَ رَوْثٍ ‏"‏‏.‏ فَأَتَيْتُهُ بِأَحْجَارٍ بِطَرَفِ ثِيَابِي فَوَضَعْتُهَا إِلَى جَنْبِهِ وَأَعْرَضْتُ عَنْهُ، فَلَمَّا قَضَى أَتْبَعَهُ بِهِنَّ‏.‏

Reference	: Sahih al-Bukhari 155
In-book reference	: Book 4, Hadith 21
USC-MSA web (English) reference	: Vol. 1, Book 4, Hadith 157