Chandogya Upanishad 8.3.5

8.3.5

tāni ha vā etāni trīṇy akṣarāṇi satīyam iti \
tad yat sat tad amṛtam \
atha yat ti tan martyam \
atha yad yaṃ tenobhe yacchati \
yad anenobhe yacchati tasmād yam \
ahar ahar vā evaṃvit svargaṃ lokam eti \\803.5\

5. Verily, these are the three syllables: sat-ti-yam. The sat (Being)—that is the immortal. The ti—that is the mortal. Now the yam—with that one holds the two together. Because with it one holds (√yam) the two together, therefore it is yam. Day by day, verily, he who knows this goes to the heavenly world.