Every time the Prophet (peace and blessings of Allah be upon him) threw a pebble at any of the three jamarat, he would say:

“Allah is the greatest.”

…on completion of the first jamarah, he advanced a little, stood facing the qiblah, raised his hands and supplicated. He also did this after the second jamarah but not the third.

Note: Jamarat are the three stoning (of the devil) areas at Mina during Hajj.

allaahu akbar