Rule 705 Federal Rules of Evidence
Rule 705 of the Federal Rules of Evidence is about Disclosing the Facts or Data Underlying an Expert. It is under Article VII (Opinions and Expert Testimony) of the Rules.
Unless the court orders otherwise, an expert may state an opinion — and give the reasons for it — without first testifying to the underlying facts or data. But the expert may be required to disclose those facts or data on cross-examination.