Bhagavad Gita · 2.58

यदा संहरते चायं कूर्मोऽङ्गानीव सर्वशः।इन्द्रियाणीन्द्रियार्थेभ्यस्तस्य प्रज्ञा प्रतिष्ठिता॥

yadā saṁharate chāyaṁ kūrmo 'ṅgānīva sarvaśhaḥ indriyāṇīndriyārthebhyas tasya prajñā pratiṣhṭhitā

When one withdraws the senses from sense objects, as a tortoise withdraws its limbs into its shell, then one's wisdom is firmly established.

— Bhagavad Gita 2.58

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