Hicks is the longest serving aide in Trump's administration. The source told ABC News the resignation will happen in the coming weeks.
ABC's Cecilia Vega reports White House press secretary Sarah Huckabee Sanders further confirmed the resignation, though, no timetable is known.
Hicks had endured nine hours of testimony on Tuesday as part of the Russia probe.
Hicks had worked under Trump a little before Trump announced his candidacy.
Hicks' move is another in long line of resignations that have happened in the White House in a little over a year of Trump's presidency.
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Here is background on Hicks:
- 29 years old
- Longest-serving member of Donald Trump's team - prior to the campaign was his longtime aide at Trump organization. Began her career in public relations.
- Named communications director in September... the youngest person ever to lead the WH communications team.
- Hicks was Trump's 4th comms director after Spicer, Dubke & Scaramucci.
- Hicks had been linked romantically with White House staff secretary Rob Porter, who resigned earlier this month. She is very close Ivanka Trump and Jared Kushner.
- She was interviewed over two days in December by Special Counsel Robert Mueller's team. Of particular interest is her role in drafting the president's statement in response to reports of the infamous Trump Tower meeting.
- She spent about nine hours interviewing behind closed doors with the House Intelligence Committee on Tuesday.
- Hicks previously worked as a model, and she was featured on one of the covers of the "Gossip Girl" book series.
- A native of Greenwich, Connecticut, she earned a B.A. from Southern Methodist University, with a major in English.