What is the balanced redox equation for hydrogen and oxygen making water?

Assume that it is in acidic medium.

The oxidation half equation: 2H₂ → 4H⁺ + 4e⁻
The reduction half equation: 4H⁺ + O₂ + 4e⁻ → 2H₂O

Add the above half equations, and cancel 4H⁺ and 4e⁻ on the both sides.
The overall equation : 2H₂ + O₂ → 2H₂O