Katha Upanishad Verse 2.3.11

Yama providing the classical definition of Yoga: the firm holding back of the senses. One must remain watchful in this state, for Yoga (the union) is subject to coming and going if the attention wavers.

Sanskrit Verse

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

Transliteration

tāṃ yogamiti manyante sthirāmindriyadhāraṇām |
apramattastadā bhavati yogo hi prabhavāpyayau || 11 ||

Translation

This firm holding back of the senses is what is known as Yoga. Then one should become watchful, for Yoga comes and goes.