CHĀNDOGYA UPANISHAD · 8.3.5

तानि ह वा एतानि त्रीण्यक्षराणि सतीयमिति
तद्यत्सत्तदमृतमथ यत्ति तन्मर्त्यमथ यद्यं तेनोभे
यच्छति यदनेनोभे यच्छति तस्माद्यमहरहर्वा
एवंवित्स्वर्गं लोकमेति ॥ ८.३.५॥

tāni ha vā etāni trīṇyakṣarāṇi satīyamiti
tadyatsattadamṛtamatha yatti tanmartyamatha yadyaṃ tenobhe
yacchati yadanenobhe yacchati tasmādyamaharaharvā
evaṃvitsvargaṃ lokameti ॥ 8.3.5॥

This name Sattyam consists of three syllables, sat-tî-yam. Sat signifies the immortal, t, the mortal, and with yam he binds both. Because he binds both, the immortal and the mortal, therefore it is yam. He who knows this goes day by day into heaven (svarga).

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)