Jabir (b. Abdullab) (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:Odd number of stones are to be used for cleaning (the private parts after answering the call of nature), and casting of pebbles at the Jamras is to be done by odd numbers (seven), and (the number) of circuits between al-Safa' and al-Marwa is also odd (seven), and the number of circuits (around the Ka'ba) is also odd (seven). Whenever any one of you is required to use stones (for cleaning the private parts) he should use odd number of stones (three, five or seven).
وَحَدَّثَنِي سَلَمَةُ بْنُ شَبِيبٍ، حَدَّثَنَا الْحَسَنُ بْنُ أَعْيَنَ، حَدَّثَنَا مَعْقِلٌ، - وَهُوَ ابْنُ عُبَيْدِ اللَّهِ الْجَزَرِيُّ - عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الاِسْتِجْمَارُ تَوٌّ وَرَمْىُ الْجِمَارِ تَوٌّ وَالسَّعْىُ بَيْنَ الصَّفَا وَالْمَرْوَةِ تَوٌّ وَالطَّوَافُ تَوٌّ وَإِذَا اسْتَجْمَرَ أَحَدُكُمْ فَلْيَسْتَجْمِرْ بِتَوٍّ " .
Muslim
The Book of Pilgrimage • Hadith 347