Verse 3.2.2.19

atha vrataṃ vratayitvā nābhimupaspṛśate | śvātrāḥ pītā bhavata yūyamāpo asmākamantarudare suśevāḥ tā asmabhyamayakṣmā anamīvā anāgasaḥ svadantu devīramṛtā ṛtāvṛdhā iti devānvā eṣa upāvartate yo dīkṣate sa devatānāmeko bhavatyanutsiktaṃ vai devānāṃ havirathaitadvrataprado mithya karoti vratamupotsiñcanvratam pramīṇāti tasyo haiṣā prāyaścittistatho hāsyaitanna mithyākṛtam bhavati na vratam pramīṇāti tasmādāha śvātrāḥ pītā bhavata yūyamāpo asmākamantarudare suśevāḥ tā asmabhyamayakṣmā anamīvā anāgasaḥ svadantu devīramṛtā ṛtāvṛdha iti sa yāvatkiyacca vrataṃ vratayitvā nābhimupaspṛśedetenaivopaspṛśetkastadveda yadvra2taprado vratamupotsiñcet

Preview of English translation:

19. Having drank the fast-food, he touches his navel, with the text (Vajasaneyi Samhita IV, 12), “Ye waters that have been drank, may ye become palatable and auspicious within us! may they prove agreeable to us, freeing us from disease and weakness and sin,—they the divine, the immortal, the holy!” Now, he who is consecrated draws nigh to the gods and becomes one of the deities; but the sacrificial food of the gods is not increased (with other material): hence, if in handing the fast-food (to the consecrated) he increases it (with other milk), he commits a fault and breaks the fast. This (formula), however, is the atonement for that (transgression), and thus that fault is not committed by him, and he does not break the fast (or  vow): therefore he says, “Ye waters . . . !” Whenever, after drinking the fast-food, he touches his navel, let him touch it with this (formula); for who knows whether (or not) he who hands the fast-food (to the consecrated) increases it (with other milk)!

For a detailled translation, including proper diacritics and footnotes, go the full English translation.