Reference for Shevuot 48:7

מהו דתימא איסור הבא מאליו אמרינן איסור הבא מעצמו לא אמרינן קמ"ל:

And R. Ashi of Avirya said in the name of R. Zera: The Tanna mentions only that for which, for wilful transgression, kareth is inflicted, but that for which, for wilful transgression, there is only a negative prohibition, he does not mention. But he mentions a guilt offering, in the case of which, for wilful transgression, there is only a negative prohibition! -

Jastrow