Jabir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Using stones (for cleaning oneself) is odd, the stoning of the Jimar is odd, the sa'y between Safa and Marwah is odd, and the tawaf is odd; so when one of you uses stones, let him use an odd number."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9321