That's probably apocryphal. The line appears in Plutarch and in Suetonius. Here's the Plutarch:

he came before the people as candidate for the priesthood, although he was not yet much more than a stripling. To this candidacy Sulla secretly opposed himself, and took measures to make Caesar fail in it, and when he was deliberating about putting him to death and some said there was no reason for killing a mere boy like him, he declared that they had no sense if they did not see in this boy many Mariuses.

And here's the Suetonius:

Everyone knows that when Sulla had long held out against the most devoted and eminent men of his party who interceded for Caesar, and they obstinately persisted, he at last gave way and cried, either by divine inspiration or a shrewd forecast: "Have your way and take him; only bear in mind that the man you are so eager to save will one day deal the death blow to the cause of the aristocracy, which you have joined with me in upholding; for in this Caesar there is more than one Marius."

There's a couple things worth noting about these passages. First, they take place at different points in Caesar's career. In Plutarch, Sulla speaks this line as justification for putting Caesar on the proscription list. In Suetonius, Sulla uses the line to wash his hands of the matter when his own party urges him to spare Caesar. The same comment moving around in the narrative doesn't lend very much confidence.

Second, the Suetonius is pretty clearly projected into the text from the perspective of someone who not only knows about the Civil War but thinks about it as the end of aristocratic government. To my knowledge that sort of view is first expressed in Tacitus, who was writing under the Flavians.

Third, there's a problem historians still struggle with: why did Sulla proscribe Caesar? Plutarch literally has people say there is no reason for killing Caesar. Both Suetonius and Plutarch talk about Sulla making Caesar divorce his wife Cornelia, who was the daughter of Sulla's enemy Cinna. Both also discuss Caesar's priesthood and Sulla's objection to it. These are both troublesome. Yeah, Caesar had family connections to Marius' party, but family connections don't necessarily mean political alliance - Sulla was a Cornelius after all. Also, the priesthood Caesar had for sure was the Flamen Dialis. This was a highly ceremonious position that basically removed the priest from political and military life, and as Lily Ross Taylor pointed out, would be the perfect place to stick Caesar if you were afraid of his political ambition. But by making Caesar divorce Cornelia Sulla forced Caesar out of that priesthood [EDIT - Caesar didn't divorce Cornelia - I was talking about Sulla's plan and not the event. Caesar seems to have never been installed as Flamen Dialis although he was selected]. Plutarch seems to have trouble with this also and had Caesar stand for some sort of elected priesthood - that couldn't be the Flamen Dialis as that wasn't an elected position. Anyway, by making Sulla say he understands Caesar's threat, these authors can explain Sulla's confusing hostility to a relative no-one.

The line moves in the narrative, it appears to be looking back from a period after the Civil Wars, and it explains an otherwise inexplicable element of the story. It's probably apocryphal.