Musnad Ahmad 319:0

Narrated to us by Hisham ibn Muhammad, from his father, from Abdullah ibn Abi Rabiah, who said: I came to the Prophet (peace be upon him) while he was in Makkah before the migration. I saw him standing, and he was saying: "O people, say: 'There is no god but Allah,' and you will succeed." Then a man came behind him and threw dust on his head and said: "This one is a man who has abandoned his religion." The Prophet (peace be upon him) did not turn around, and he continued saying: "O people, say: 'There is no god but Allah,' and you will succeed."