Sahih Bukhari Volume 4, Book 52, Hadith Number 148.

Narated By Salama bin Al-Akwa : The Prophet passed by some people of the tribe of Bani Aslam who were practicing archery. The Prophet said, “O Bani Ismail ! Practice archery as your father Isma’il was a great archer. Keep on throwing arrows and I am with Bani so-and-so.” So one of the parties ceased throwing. Allah’s Apostle said, “Why do you not throw?” They replied, “How should we throw while you are with them (i.e. on their side)?” On that the Prophet said, “Throw, and I am with all of you.”