What to actually say
Open with this, more or less word for word: Before you ask me again, I need to tell you where I've landed on lending money, and it's not what you're hoping to hear.
I'm not going to lend you money anymore. Not because of you specifically, but because I've figured out I'm bad at being someone's bank and I'd rather just be your friend.
It names the real trade-off instead of blaming him, which makes the no about a decision rather than an accusation.
The last two times I lent you money, I told myself it was fine, and then I spent months quietly annoyed at you without ever saying so. I don't want to do that to either of us again.
Using specific past evidence is harder to argue with than a general principle, and it shows the cost was real, not hypothetical.
If you're actually in trouble, I'll sit with you and look at the numbers, or help you figure out who to call about a payment plan. I just can't be the money.
Offering a concrete alternative keeps the door open to helping without keeping the door open to lending.
I know this might make things weird between us for a while, and I'd rather have that than keep lending you money I'm scared to ask for back.
Saying the cost out loud is more credible than pretending the friendship will just absorb it painlessly.
How it usually goes
A version of the conversation with the push-back left in, because that is the part you need to have already heard once.
What not to say
I wish I could, but I just don't have it right now.
This blames your bank account instead of your decision, so he'll simply ask again next payday.
You know I would if I could.
It signals willingness under different conditions, which is exactly the maybe he's listening for.
Let me think about it.
It buys you nothing but time, and time is what turns a no into a slow yes.
If he says you're being cold or that this changes things between you
Don't rush to soften it or take it back. Let it sit for a second, then say: "I hear that this stings, and I'd rather you be upset with me for a while than keep pretending I'm fine lending money I'm not fine lending." You're not trying to win the moment, you're trying to hold the line without escalating into a fight about loyalty. If he keeps pushing on the same point, repeat the decision in fewer words each time rather than defending it with new reasons — new reasons just give him new angles to argue against.