Midrasch zu Dewarim 17:4

"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).

(Devarim 17:4) "And it be told to you, and you hear, and you search it out well": And elsewhere (Ibid. 13:15) it is written "And you shall inquire, and you shall search out, and you shall ask well," and elsewhere (Ibid. 19:18) "And the judges shall inquire well." "well" - "well," for a gezeirah shavah (identity). We are hereby taught that he is examined with seven chakiroth (cross-examinations). This tells me only of chakiroth. Whence do I derive (the same for) bedikoth (examinations)? From (13:15, 17:4) "and, behold, true and correct is the thing."