The Passover Letter of Elephantine is the oldest known ex-biblical account of Passover laws. Sent by Hananiah, a high official in Jerusalem, to his “brother” Yadaniyah and the Judean soldiers on Egypt’s Elephantine island, the papyri are 400 years older than the Dead Sea Scrolls.
https://www.israpundit.org/2400-year-old-passover-letter-shows-evolution-of-jewish-ritual/