gettxoutproof ["txid",...] ( blockhash )

Returns a hex-encoded proof that "txid" was included in a block.

NOTE: By default this function only works sometimes. This is when there is an
unspent output in the utxo for this transaction. To make it always work,
you need to maintain a transaction index, using the -txindex command line option or
specify the block in which the transaction is included manually (by blockhash).

Return the raw transaction data.

1. "txids" (string) A json array of txids to filter
"txid" (string) A transaction hash
2. "block hash" (string, optional) If specified, looks for txid in the block with this hash

"data" (string) A string that is a serialized, hex-encoded data for the proof.
Don't trust. Verify. Docs extracted from src/rpc/rawtransaction.cpp#L222-L242.