Use the dumpwallet RPC command. Do bitcoin-cli dumpwallet where is the name of the file you want the wallet data to be written to. Open up that file as a regular text file. At the top of it will be something that has the master private key (an xprv). Somewhere in the file will be a private key with the hdkeypath listed as "m". That is the seed. Note that the seed is not a phrase; Bitcoin Core does not use mnemonics (which are often misnamed as seeds).