Trump ended months of speculation on whether he would respond to questions from the special counsel, who is investigating whether the president's 2016 election campaign colluded with Russians and if Trump illegally obstructed the probe.
"I've answered them very easily".
Trump's team agreed to have the president submit written answers to questions relating to Russia's role in the election, but it is still possible Mueller may press Trump to answer questions on potential obstruction at some point.
After denying he was agitated, Trump said Friday in a reporter's followup question: "I like to take everything personally, because you do better that way".
"You always have to be careful when you answer questions from people who probably have bad intentions", he added.
"As you know, I've been very busy", he quipped, bringing up last week's trip to Europe and several trade deals that were recently negotiated.More news: Deepika, Ranveer wedding: Glamour, luxury and a bit of showbiz drama
More news: Stormy Daniels' Attorney Michael Avenatti Arrested for Felony Domestic Violence
More news: Fortnite Among The 15 Xbox Games To Support Mouse & Keyboard Inputs
"There should have never been any Mueller investigation" he said Friday. "I write answers", he said.
The source would not say if the concerns have been conveyed to Mueller's team.
Trump said that while he had finished the questions, he had not turned them into the special counsel yet.
Trump has called the probe a "witch hunt" and said there was no such collusion. "I imagine it's ending now", he said.
Trump's lawyers counseled him strongly not to agree to sit down with Mueller and investigators in person, although the president has never publicly ruled out that he might do so. "They have found no collusion and have gone absolutely nuts", Trump tweeted.
And Mueller's office asked a federal court in Washington, DC, for an extension on the sentencing of Rick Gates, the former Trump campaign deputy chairman, until January - a sign that Gates still has a significant amount of information for prosecutors.
Tensions have risen even higher over the probe since Trump last week ousted Jeff Sessions as USA attorney general and named Matthew Whitaker as his replacement on an acting basis.