KAṬHA UPANISHAD · 2.3.11

तां योगमिति मन्यन्ते स्थिरामिन्द्रियधारणाम् ।
अप्रमत्तस्तदा भवति योगो हि प्रभवाप्ययौ ॥ ११॥

tāṃ yogamiti manyante sthirāmindriyadhāraṇām ।
apramattastadā bhavati yogo hi prabhavāpyayau ॥ 11॥

The state unperturbed when the senses are imprisoned in the mind, of this they say “it is Yoga”. Then man becomes very vigilant, for Yoga is the birth of things and their ending.