מדרש על סנהדרין 79:6

"And you shall inquire": of the Torah. "and you shall search out": from the witnesses. "and you shall ask": of the Talmud. Here (in respect to a man's serving idolatry) it is written (Ibid. 17:4) "And it be told to you, and you hear, and you inquire well," and elsewhere (in respect to scheming witnesses) (19:18) "And the judges inquire well" — "well" - "well" for an identity (gezeirah shavah.) (We are hereby taught) that he (the witness) undergoes seven cross-examinations (chakiroth).