Why is the lamentation for the Negus considered among the proofs of prophethood?

The Messenger of Allah, peace and blessings be upon him, mourned the Negus on the day he died, saying: "Today, a righteous servant of Allah, As-hamah, has died." This is one of the proofs of prophethood, because knowing this on the same day, from Abyssinia to Medina, was not possible through ordinary means of communication. Thus, his knowledge of this came through divine revelation.