CC Adi 7.129

sarvāśraya īśvarera praṇava uddeśa
‘tat tvam asi’—vākya haya vedera ekadeśa

sarva-āśraya—the reservoir of everything; īśvarera—of the Supreme Personality of Godhead; praṇava—oṁkāra; uddeśa—purpose; tat tvam asi—the Vedic mantra tat tvam asi (“you are the same”); vākya—statement; haya—becomes; vedera—of the Vedic literature; eka-deśa—partial understanding.

“It is the purpose of the Supreme Personality of Godhead to present praṇava [oṁkāra] as the reservoir of all Vedic knowledge. The words ‘tat tvam asi’ are only a partial explanation of the Vedic knowledge.

Tat tvam asi means “you are the same spiritual identity.”