...the deva spoke this verse...:

Hard it is to keep, and hard to bear, Recluse-life for him who lacks the skill. Obstacles abound, the fool is lost. How long can he endure the holy life, If he cannot hold his heart in check? Caught now here, now there, he stumbles, falls,

[The Blessed One replied:]

As the tortoise draws into his shell Each limb, the monk, withdrawn, with mind applied, Unattached, and doing harm to none, Passions wholly stilled, dwells blaming none.[1]

Notes

1.: Cf. Bhagavadgiitaa ii, 58: "He who withdraws his senses on all sides from sense-objects as the tortoise draws in his limbs, is firmly established in wisdom." The same image occurs in SN 35.199.