Exodus 4:23

23.And I say unto thee, Let my son go. This was not the beginning of the legation, but its final clause; for Moses warned the desperate man of his son’s death, when everything else had been tried in vain. The meaning is, then, that the obstinacy of the tyrant must not prevent Moses from pressing him even to this final act. Therefore this injunction was an exhortation to perseverance; as appears from the context, when God declares that he will punish the obstinacy of the tyrant, because he refused to obey the command to let the people go. Moreover, since this denunciation was very severe, and might very greatly awaken the tyrant’s wrath, therefore Moses is thus early commanded to prepare himself lest he should fail in this particular.