Herodotus:
...there was war between the Lydians and the Medes for five years; each won many victories over the other, and once they fought a battle by night. They were still warring with equal success, when it happened, at an encounter which occurred in the sixth year, that during the battle the day was suddenly turned to night. Thales of Miletus had foretold this loss of daylight to the Ionians, fixing it within the year in which the change did indeed happen. So when the Lydians and Medes saw the day turned to night, they stopped fighting, and both were the more eager to make peace. Those who reconciled them were Syennesis the Cilician and Labynetus the Babylonian; they brought it about that there should be a sworn agreement and a compact of marriage between them: they judged that Alyattes should give his daughter Aryenis to Astyages, son of Cyaxares; for without strong constraint agreements will not keep their force. These nations make sworn compacts as do the Greeks; and besides, when they cut the skin of their arms, they lick each other's blood1.
See:
- Herodotus, Histories, 1.73-74
- Kevin Leloux, The Battle of the Eclipse (May 28, 585 BC): A Discussion of the Lydo-Median Treaty and the Halys Border. In: Polemos. Volume 19, no. 2, 2016, pp. 31–54