Shatapatha Brahmana 9.5.1.9

9.5.1.9

sa trīntstanānupaiti | tatparāpaśyati sa dvau stanā upaiti tannedīyasaḥ parāpaśyati sa ekaṃ stanamupaiti tadadhigacati na tvabhipattuṃ śaknoti sa upavasathe 'nāśakamupaityetadvai sarvaṃ tapo yadanāśakastasmādupavasathe nāśnīyāt

He resorts to three teats (of the cow): he sees it. He resorts to two teats: he sees it nearer by. He resorts to one teat: it comes nigh to him, but he cannot lay hold of it. He undergoes entire abstinence from food; for the whole (practice of) penance it is when one abstains from food: let him therefore eat nothing on the day of preparation.