Bhagavad Gita Chapter 2.58
yadā saḿharate cāyaḿ
kūrmo ‘ńgānīva sarvaśaḥ
indriyāṇīndriyārthebhyas
tasya prajñā pratiṣṭhitā
Translation
One who is able to withdraw his senses from sense objects, as the tortoise draws its limbs within the shell, is firmly fixed in perfect consciousness.