What did Moses write down and teach the Israelites shortly before his death?

Answer: A song

Moses wrote down a song that very day and taught it to the Israelites, so it would be a witness for God against them.

“Now therefore, write down for yourselves this song and teach it to the Israelites; have them recite it, so that it may be a witness for Me against them. So that very day Moses wrote down this song and taught it to the Israelites.”— Deuteronomy 31:19, 31:22 (BSB)