Rashi on Deuteronomy 21:9/רש"י על דברים כ״א:ט׳

1ואתה תבער BUT THOU SHALT PUT AWAY [THE GUILT OF INNOCENT BLOOD FROM AMONG YOU] — This teaches that if the murderer is found after the heifer’s neck was broken he must nevertheless be put to death, — and this is what Scripture describes as הישר בעיני ה׳ RIGHT IN THE EYES OF THE LORD (cf. Sotah 47b; Ketubot 37b and Tosafot on Ketubot 37b:16.1 ואח"כ נמצא ההורג).