A system has a time independent Hamiltonian with an eigenbasis $|E_i\rangle$. Show that the probability of getting $E_k$ when you measure for the energy is time independent.
So if I let a system $|\psi\rangle=\sum a_i|E_i\rangle$ the probability of getting $E_k$ is:
$$ P=\langle E_ k|\psi\rangle\langle\psi|E_k\rangle=|\langle E_ k|\psi\rangle|^2 $$
and I will use the relations: $$ \frac{d}{dt}|\phi\rangle=-i/\hbar H |\phi\rangle $$ and its dual $$ \frac{d}{dt}\langle\phi|=i/\hbar\langle\phi|H $$
If we differentiate the probability using the product rule:
$$ \frac{dP}{dt}= \langle \frac{d}{dt}E_ k|\psi\rangle\langle\psi|E_k\rangle +\langle E_ k|\frac{d}{dt}\psi\rangle\langle\psi|E_k\rangle +\langle E_ k|\psi\rangle\langle\frac{d}{dt}\psi|E_k\rangle +\langle E_ k|\psi\rangle\langle\psi|\frac{d}{dt}E_k\rangle $$
$$ =i/\hbar( \langle E_ k|H|\psi\rangle\langle\psi|E_k\rangle- \langle E_ k|H|\psi\rangle\langle\psi|E_k\rangle+ \langle E_ k|\psi\rangle\langle\psi|H|E_k\rangle- \langle E_ k|\psi\rangle\langle\psi|H|E_k\rangle ) $$ $$ =0 $$
The second equality uses the relationships given above.
This proof, however, doesn't use the fact that the Hamiltonian is time independent and so I think I must have missed something.