Bhagavatapurana 2.9.6

2.9.6

sa cintayan dvyakṣaram ekadāmbhasy upāśṛṇoddvirgaditaṃ vaco vibhuḥ /
sparśeṣu yat ṣoḍaśam ekaviṃśaṃ niṣkiñcanānāṃ nṛpa yaddhanaṃ viduḥ //

6. While thus revolving thoughts regarding creation (while contemplating the methods of creation), Brahma the great master once heard (uttered) very near him a word of two syllables (_Tapa_) repeated twice over from under water; the two letters forming the word are the sixteenth and the twenty-first of the five classes of consonants—the word which is considered, O king, the wealth of those who possess no (have relinquished the) wealth of gold and silver.