A crime was committed at baker street. Ibrahim Dakota who was shot at stomach was the main suspect. Sherlock questioned the suspect. Conversation started as:
Sherlock: Whats your story Ibrahim?
Ibrahim: I was walking around the baker street and suddenly a man from back shot me. I ran as fast as he could to save my life".
Sherlock: That is enough (and ask the police to arrest him).

Why sherlock things he is the murderer?

If Shot from back, then how can he get the injury at the stomach. Therefore a fake story.