Sunan Ibn Majah, Chapter: The Chapters on Supplication

that the Messenger of Allah (saw) said: "Whoever unexpectedly comes across a person suffering a calamity, and says: Al-hamdu Lillahil-ladhi 'afani mim-mabtalaka bihi, wa faddalani 'ala kathirin mimman khalaqa tafdila (Praise is to Allah Who has kept me safe from that which has afflicted you and preferred me over many of those whom He has created), will be kept safe from that calamity, no matter what it is.'"