Look up a transaction by its hash. This is the same hash that is returned by the API when submitting a transaction (see PendingTransaction).
When given a transaction hash, the server first looks for the transaction in storage (on-chain, committed). If no on-chain transaction is found, it looks the transaction up by hash in the mempool (pending, not yet committed).
To create a transaction hash by yourself, do the following:
SHA3-256 to the hash message bytes.0x prefix.