Musnad Ahmad 1153:2189

Rawh narrated to us, he said: Ibn Jurayj narrated to us, he said: Abdullah ibn Abi Mulaykah informed me, from al-Qasim ibn Muhammad, that he informed him repeatedly, that Aisha informed him that the Prophet, peace and blessings of Allah be upon him, used to say: "No thorn, or anything beyond it, afflicts a Muslim, except that it is an expiation for him."