400
The Holy Prophet (saw) wrote letters to various Kings, such as the Roman Emperor, King of Iran, and King of Abyssinia.
Of these three, which King gave the most welcoming response?
King of Abyssinia, Negus. He showed great regard and respect for it. He held it up to his eyes, descended from the throne and ordered an ivory box for it. Then he deposited it in the box and said "While this letter is safe, my kingdom is safe".