Mandukya Karika, verse 3.41

Text

उत्सेक उदधेर्यद्वत्कुशाग्रेणैकबिन्दुना ।
मनसो निग्रहस्तद्वद्भवेदपरिखेदतः ॥ ४१ ॥

utseka udadheryadvatkuśāgreṇaikabindunā |
manaso nigrahastadvadbhavedaparikhedataḥ || 41 ||

41. The mind can be brought under control only by an unrelenting effort like that which is required to empty an ocean, drop by drop, with the help of a (blade of) Kuśa- grass.

Shankara Bhashya (commentary)

As one may try to empty the ocean, by draining off its water drop by drop, with the help of a (blade of) Kuśa -grass, even so may one control the mind by making the same effort with a heart which becomes neither1 depressed nor tired. This Kārikā gives us an idea of the effort that a Yogi should make to control his mind completely. But it appears that the complete suppression of the mental Vṛttis is impossible in this way. And as the happiness of a Yogi is dependent upon such suppression, he can never attain to eternal Truth by the Yogic method. Jñāna - yoga is the royal road for the attainment of eternal Truth and peace.