Sahih Bukhari, Chapter: Jihaad (Fighting for the cause of Allah)

The Prophet (saw) passed by some people of the tribe of Bani Aslam who were practising 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 (SAW) said, "What is the matter with you! Why have you ceased throwing?" 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."