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