Sahih Muslim Book 7, Hadith Number 2982.

Chapter : What should be the number of the pebbles?

Jabir (b. Abdullah) (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) 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).