Why is the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ given by $E=-p_av^a$?

Question

On page 61 of his book “General Relativity”, Robert Wald says that the energy of a particle with $4$-momentum $p^a = mu^a$ measured by an observer with $4$-velocity $v^a$ is given by $E=-p_av^a$. How can I see that?