Bhagavad Gita · 2.41

व्यवसायात्मिका बुद्धिरेकेह कुरुनन्दन।बहुशाखा ह्यनन्ताश्च बुद्धयोऽव्यवसायिनाम्॥

vyavasāyātmikā buddhir ekeha kuru-nandana bahu-śhākhā hy anantāśh cha buddhayo 'vyavasāyinām

O Arjuna, the resolute intellect is one-pointed, but the intellects of the irresolute are many-branched and endless.

— Bhagavad Gita 2.41

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