Musannaf Ibn Abi Shayba 21:532

'Abd al-Rahman ibn Mahdi narrated to us, from Mughira, who said: I said to Ibrahim, "What should I say when I throw the pebble?" He said: "Say, 'O Allah, make it an accepted Hajj and a forgiven sin.'" He said: "Should I say it with every pebble?" He said: "Yes, if you wish."