grain of truth
element of truth

But there is at least a grain of truth in what he says.

Perhaps there was a grain of truth in the story.

However, the story may still, in part, contain some grain of truth.

If there hadn't been a grain of truth in what he said, it couldn't have hurt so much.

"There is not a grain of truth to these words."

But that grain of truth in what he had told her would surface somewhere.

And there is more than a grain of truth in this.

There is a grain of truth in what you say, but no more.

Oh, I admit there's a grain of truth in almost everything you say.

However in this case I think that there may well be a grain of truth to it.