He said it would be up to his successor to negotiate the Brexit.
He made the comment after attending his last European Council summit in Brussels last night.
The talks were described as sombre and constructive.
David Cameron told EU leaders the UK voted to leave the union because people feel there's no control over immigration from Europe.
But Mr Cameron said he'd continue pressing for close relations between the UK and the EU:
Meanwhile EU Commission President has hit out at Britain's Leave campaign.
He's accusing them of having no plan for the future of the UK, once it leaves the European Union.
Jean Claude Juncker says the pro-Brexit camp don't actually know what they want:
