Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 20

Sanskrit:

मनोगतिं न विसृजेज्जितप्राणो जितेन्द्रिय: ।सत्त्वसम्पन्नया बुद्ध्या मन आत्मवशं नयेत् ॥ २० ॥

ITRANS:

mano-gatiṁ na visṛjejjita-prāṇo jitendriyaḥsattva-sampannayā buddhyāmana ātma-vaśaṁ nayet

Translation:

One should never lose sight of the actual goal of mental activities, but rather, conquering the life air and senses and utilizing intelligence strengthened by the mode of goodness, one should bring the mind under the control of the self.

Purport:

Although the mind may suddenly wander outside the jurisdiction of self-realization, one must bring the mind back under control by clear intelligence in the mode of goodness. The best solution is to keep the mind always busy in the service of Lord Kṛṣṇa so that the mind cannot wander onto the dangerous path of sense gratification, headed by sex attraction. The material mind is naturally inclined to accept material objects at every moment. Therefore unless the mind is seriously brought under control there is no possibility of becoming steady on the path of spiritual advancement.