Shatapatha Brahmana 14.1.1.28

14.1.1.28

tisro rātrīrvrataṃ carati trayo vā ṛtavaḥ samvatsarasya samvatsara eṣa ya eṣa tapatyeṣa u pravargyastadetamevaitatprīṇāti tasmāttisro rātrīrvrataṃ carati

For three nights he keeps the rule (of abstinence); for there are three seasons in the year, and the year is he that shines yonder, and the Pravargya also is that one: it is him he thereby gratifies, and therefore he keeps the rule for three nights.