Shatapatha Brahmana 4.5.5.6

4.5.5.6

atha yadantaryāmaṃ hutvā | avāñcamavamārṣṭi tasmādimā avayo 'vācīnaśīrṣṇyaḥ khanantya iva yantyetā vai prajāpateḥ pratyakṣatamāṃ yadajāvayastasmādetāstriḥ saṃvatsarasya vijāyamānā dvau trīniti janayanti

And because, having offered the Antaryâma, he wipes (the vessel) downwards, therefore these sheep walk with their heads down, as if digging. Now they, the goats and sheep, are most conspicuously Pragâpati-like: whence, bringing forth thrice in the year, they produce two or three (young ones).