Musnad Ahmad 1153:2227

Yunus narrated to us, Fulayh narrated to us, from Abd al-Rahman ibn al-Qasim, from his father, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is nothing that afflicts a Muslim, not even a thorn that pricks him, but that it diminishes his sins."