\[ y''(x)-y(x) \left (f'(x)+f(x)^2\right )=0 \] ✗ Mathematica : cpu = 0.146941 (sec), leaf count = 0 , could not solve
DSolve[-(y[x]*(f[x]^2 + Derivative[1][f][x])) + Derivative[2][y][x] == 0, y[x], x]
✓ Maple : cpu = 0.142 (sec), leaf count = 22
\[ \left \{ y \left ( x \right ) = \left ( \int \!{{\rm e}^{\int \!-2\,f \left ( x \right ) \,{\rm d}x}}\,{\rm d}x+{\it \_C1} \right ) {{\rm e}^{\int \!f \left ( x \right ) \,{\rm d}x}}{\it \_C2} \right \} \]