Sahih Bukhari Volume 7, Book 71, Hadith Number 632.
Narrated By Abu Said Al-Khudri : Some of the companions of the Prophet came across a tribe amongst the tribes of the Arabs, and that tribe did not entertain them. While they were in that state, the chief of that tribe was bitten by a snake (or stung by a scorpion). They said, (to the companions of the Prophet), “Have you got any medicine with you or anybody who can treat with Ruqya?” The Prophet’s companions said, “You refuse to entertain us, so we will not treat (your chief) unless you pay us for it.” So they agreed to pay them a flock of sheep. One of them (the Prophet’s companions) started reciting Surat-al-Fatiha and gathering his saliva and spitting it (at the snake-bite). The patient got cured and his people presented the sheep to them, but they said, “We will not take it unless we ask the Prophet (whether it is lawful).” When they asked him, he smiled and said, “How do you know that Surat-al-Fatiha is a Ruqya? Take it (flock of sheep) and assign a share for me.”