Shiva Purana
ओमितीदं सर्वमिति सर्वं ब्रह्मेति च श्रुतेः ॥ वाच्यवाचकसम्बन्धोप्ययमेवार्थ ईरितः ॥ ३३॥
Om is everything, everything is Brahman. (Om Itīdam Sarvam Iti Sarvam Brahma) This Vedic Text establishes the link between the word and its meaning.
english translation
omitIdaM sarvamiti sarvaM brahmeti ca zruteH ॥ vAcyavAcakasambandhopyayamevArtha IritaH ॥ 33॥
hk transliteration by Sanscript