Question: What is the ruling regarding washing the stones for stoning (the Jamaraat)?

Response:
They should not be washed. Rather, if a person washes them with the intention of worshipping Allaah (by this action), then this is an innovation because the Prophet (sal-Allaahu `alayhe wa sallam) did not do this.

Shaykh Ibn 'Uthaymeen
al-Bid'u wal-Muhdathaat wa maa laa asla lahu - Page 404

Taken from fatwa-online.com