Agniveshyagrihyasutra 2.4.12.8

2.4.12.8

upamārahitāḥ aumakārātmakāḥ kālatrayātmakāḥ lokatrayātmakāḥ ṛgyajussāmātmakāḥ avasthātrayātmakāḥ praṇavoccāraṇena mahadaiśvaryadāyakāḥ mūlādhārānāhatabhrūmadhye sthitāḥ akārokāramakārāḥ udāttānudāttasvaritāḥ vakṣyāmy tirahasyamiti \

Incomparable, consisting of the sacred syllable Om, consisting of the three times, consisting of the three worlds, consisting of the Ṛg, Yajus, and Sāma, consisting of the three states, granting great sovereignty by the utterance of the sacred syllable Om, abiding in the Mūlādhāra, Anāhata, and between the eyebrows—the phonemes A, U, and M, possessing acute, grave, and circumflex accents: "I shall proclaim the supreme secret."