Bg 6.26

yato yato niscalati
manas cancalam asthiram
tatas tato niyamyaitad
atmany eva vasam nayet

Word for word: 
yatah yatah — wherever; niscalati — becomes verily agitated; manah — the mind; cancalam — flickering; asthiram — unsteady; tatah tatah — from there; niyamya — regulating; etat — this; atmani — in the Self; eva — certainly; vasam — control; nayet — must bring under.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
From wherever the mind wanders due to its flickering and unsteady nature, one must certainly withdraw it and bring it back under the control of the Self.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The nature of the mind is flickering and unsteady. But a self-realized yogi has to control the mind; the mind should not control him. One who controls the mind (and therefore the senses as well) is called gosvami, or svami, and one who is controlled by the mind is called go-dasa, or the servant of the senses. A gosvami knows the standard of sense happiness. In transcendental sense happiness, the senses are engaged in the service of Hrsikesa, or the supreme owner of the senses – Krishna. Serving Krishna with purified senses is called Krishna consciousness. That is the way of bringing the senses under full control. What is more, that is the highest perfection of yoga practice.