When I send a transaction with Electrum Wallet, -snip-
Bob sends 1 Bitcoin to Mary on Electrum Wallet
Who does Mary think the Bitcoin came from? What address does she think is Bob's?
So you're Bob in this scenario, then the answer is the second part of my original reply.
In Electrum, it's in the transaction details' list of inputs.
If Mary is also using Electrum, she wouldn't have to guess since her receiving addresses should have green highlight,
the non-highlighted address(es) should be Bob's unless he's used some sort of coin control, CoinJoin or custodial wallets, etc.
In Bob's Electrum, he'll also see highlighted addresses (
green: receiving / yellow: change) that'll differentiate his receiving and change addresses from the unrelated ones.