It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever performs ablution should clean his nose by blowing it out, and whoever uses stones (to clean himself) should use an odd number."
(b) Another narration from Sayyiduna Abu Hurayrah (may Allah be pleased with him) is reported likewise.
(c) From Sayyiduna Jabir (may Allah be pleased with him) the same is reported concerning the use of stones.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 504
It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you uses stones (to clean himself), let him use three."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 505
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Whoever uses stones should use an odd number. Whoever does this has done well, and whoever does not, there is no harm (on him)."
(b) If this narration is authentic, then the odd number applies after three. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 506
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When one of you uses stones, let him use an odd number, for Allah Almighty is odd (Witr) and loves what is odd. Do you not see that the heavens are seven, the earths are seven and the circuits (of tawaf) are seven?" And he mentioned other things besides these.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 507