Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.4.27

3.4.4.27

tapasā vai lokaṃ jayanti | tadasyaitatparaḥ para eva varīyastapo bhavati paraḥparaḥ śreyāṃsaṃ lokaṃ jayati vasīyānu haivāsmiṃloke bhavati ya evaṃ vidvānparo 'hvīrupasada upaiti tasmādu paro 'hvīrevopasada upeyādyadyu dvādaśopasada upeyāttrīṃścaturahaṃ dohayeddvau caturahamekaṃ caturaham

Verily, the world is conquered by austere devotion. Now, his devotion becomes ever and ever wider, he conquers an ever and ever more glorious world and becomes better even in this world, whosoever, knowing this, undertakes the Upasads that get narrower and narrower: let him, therefore, undertake the Upasads that get narrower and narrower. And should he undertake twelve Upasads, let him have three (teats) milked out for four days, two for four days, and one for four days.