Reference for Chullin 281:38

כל מצות לא תעשה שיש בה קום עשה אין לוקין עליה:

<big><b>MISHNAH: </b></big>IF A MAN TOOK THE DAM WITH THE YOUNG, R'JUDAH SAYS, HE HAS INCURRED [FORTY] STRIPES, AND HE NEED NOT NOW LET HER GO.