That would be Amarna Letters 41-44.

A quotation from a letter sent by Ankhesenamun, Tutankhamun's widow, to Hittite king Šuppiluliuma I was found in Ḫattuša. Ankhesenamun asked Šuppiluliuma to send her one of her sons to marry her and become king of Egypt. Šuppiluliuma initially thinks this is a ruse, but the story eventually checks out and Šuppiluliuma sends a son. That son is killed on his journey and a war between Egypt and Ḫattuša ensues.