The Chandogya Upanishad belongs to the Samaveda, and in it Rishi Uddalaka instructs his son Shvetaketu through images drawn from ordinary things — a seed opened, salt dissolved in water — each returning to the same recognition. The instruction closes with the sentence the tradition carries as tat tvam asi. Its earlier chapters take up the udgitha and the syllable Om.
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In the Chandogya Upanishad, Rishi Uddalaka asks his son to dissolve salt in water and then find it again. He cannot — but every sip is salt. That demonstration ends with the words Tat Tvam Asi.
Source: Chandogya Upanishad 6.13