\hat{U}(dt)\hat{A}(t)\hat{U}^{-1}(dt)=e^{-\frac{i}{\hbar}\hat{H}dt}\hat{A}(t)e^{\frac{i}{\hbar}\hat{H}dt}=(\hat{I}-\frac{i}{\hbar}\hat{H}dt) \hat{A}(t)(\hat{I}+ \frac{i}{\hbar}\hat{H}dt)