In a particular text (not able to remember) there was a line which said that :
A Brahmin, a woman and a cow must never be slain.
This statement is in line with the pancha mahapataka among which, is the killing of a Brahmin and a sin equivalent to it, which is the killing of a woman. However according to some Dharmashastras (Manu and Parasara go into some more details), the killing of a cow is categorised as an Upapataka something on level (may be slightly graver but it’s still categorised under Upapataka) with any other killing, which is surprising to note, considering the holiness of the cow and prescription against its killing even in Vedic texts.
Besides the Vedic support against cow killing, in another text, it has been stated that hearing vilification of God is equivalent to the sin of killing a cow. I’m sure that vilifying or hearing thus about God should be one of the gravest of sins and not merely an Upapataka, therefore-
Is there any evidence from any of the scriptures to show that the killing of a cow is a Mahapataka, equivalent to the killing of a Brahmin or a woman?
The answer may not necessarily be from Dharmashastras although those are preferable opposed to a single line from Itihasa Purana. Additionally, it would be nice to have information on any pre-colonial versions or even quotes in Smriti compendium books in this regard (similar to quotes about women being allowed Upanayanam).
Incidentally if a reference to the vilification of God being a Mahapataka is found too, it would be a nice accompaniment.