Mudgalapurana 3.32.37

3.32.37

tatastaṃ pratyuvācedaṃ vacanaṃ caturānanaḥ |
pūrvajanmani sambhūtaṃ dvijahatyāyutaṃ vratam ||

Then the Four-faced One spoke these words to him: / “A vow joined with the murder of twice-born [brahmins] was performed in his previous life.”