Reference for Chullin 4:2

הַמְבַכֶּרֶת הַמַּקְשָׁה לֵילֵד, מְחַתֵּךְ אֵבָר אֵבָר וּמַשְׁלִיךְ לַכְּלָבִים. יָצָא רֻבּוֹ, הֲרֵי זֶה יִקָּבֵר, וְנִפְטְרָה מִן הַבְּכוֹרָה:

[For] an animal pregnant with its firstborn which is having difficulty giving birth, one [may] cut limb by limb [from the fetus] and cast it to the dogs. [If] the majority of it protruded, it must be buried and [the mother] is exempt from [laws regarding] the firstborn.

Jastrow