The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 866
Jabir b. Samura reported: When we said prayer with the Messenger of Allah (may peace be upon him), we pronounced: Peace be upon you and Mercy of Allah, peace be upon you and Mercy of Allah, and made gesture with the hand on both the sides. Upon this the Messenger of Allah (may peace be upon him said: What do you point out with your hands as if they are the tails of headstrong horses? This is enough for you that one should place one's hand on one's thigh and then pronounce salutation upon one's brother on the right side and then on the left.