Shatapatha Brahmana 6.3.1.43

6.3.1.43

tāṃ caturbhirādatte | caturakṣarā vai sarvā vāgvāgityekamakṣaramakṣaramiti tryakṣaraṃ tadyattadvāgityekamakṣaraṃ yaivaiṣānuṣṭubuttamā sā sātha yadakṣaramiti tryakṣarametāni tāni pūrvāṇi yajūṃṣi sarvayaivaitadvācāgniṃ khanati sarvayā vācā sambharati tasmāccaturbhiḥ

He takes it up with four (formulas), for all speech consists of four syllables: 'vâk' (speech) is one syllable, and 'aksharam' (syllable) consists of three syllables. Now that monosyllable 'vâk' is the same as this last one, the Anushtubh; and that trisyllable 'aksharam' is the same as those former formulas: he thus digs up Agni by the whole speech, and equips it with the whole speech,--hence with four (formulas).