Bhagavad Gita · 6.33

अर्जुन उवाचयोऽयं योगस्त्वया प्रोक्तः साम्येन मधुसूदन।एतस्याहं न पश्यामि चञ्चलत्वात्स्थितिं स्थिराम्॥

arjuna uvācha yo 'yaṁ yogas tvayā proktaḥ sāmyena madhusūdana etasyāhaṁ na paśhyāmi chañchalatvāt sthitiṁ sthirām

Arjuna said: O Madhusudana, this yoga of equanimity that You have described — I do not see how it can be maintained, given the restlessness of the mind.

— Bhagavad Gita 6.33

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)