Verse 9.5.2.9

aṣṭarcenopatiṣṭhetetyu haika āhuḥ | vayaṃ te adya rarimā hi kāmamiti dvitīyayā kāmavatyā sapta pūrvāstadaṣṭāvaṣṭākṣaro gāyatrī gāyatro'gniryāvānagniryāvatyasya mātrā tāvataivāsya tadāpnoti yadasya kiṃ cānāptamatho evaṃ samaṃ devate bhajete iti na tathā kuryādetā vāva saptāṣṭāvanuṣṭubho bhavanti sa yo'ṣṭarce kāmo'traiva tamāpnoti

Preview of English translation:

9. “Let him approach (the fire-altar) with an eight-versed (hymn)!” say some;—“with (Vajasaneyi Samhita XVIII, 75), "We thereby offer unto thee thy wish, reverently approaching thee with open hands: with holiest mind and peaceful thought offer thou sacrifice unto the gods as priest, O Agni!" thus with a second wish-holding one,—and the seven foregoing ones, that makes eight,—the Gayatri consists of eight syllables, and Agni is of Gayatra nature: as great as Agni is, as great as is his measure, by so much he thus secures for him whatsoever is not yet secured for him; and thus, moreover, the two deities receive the same (amount) for their share.” Let him, however, not do so, for surely those seven (verses) are (equal to) eight anushtubh (verses), and thus he even therein obtains whatever wished-for object there is in the eight-versed (hymn).

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