Shiva Purana
तद्बीजं सर्वविद्यानां मंत्रमाद्यं षडक्षरम् ॥ अतिसूक्ष्मं महार्थं च ज्ञेयं तद्वटबीजवत् ॥ ७ ॥
The first mantra consisting of six syllables is the seed of all lores. It is very subtle but serves a great purpose. It shall be known like the seed of the banyan tree.
english translation
tadbIjaM sarvavidyAnAM maMtramAdyaM SaDakSaram ॥ atisUkSmaM mahArthaM ca jJeyaM tadvaTabIjavat ॥ 7 ॥
hk transliteration by Sanscript