ŚVETĀŚVATARA UPANISHAD · 2.10

समे शुचौ शर्करावह्निवालुका- विवर्जिते शब्दजलाश्रयादिभिः ।
मनोऽनुकूले न तु चक्षुपीडने गुहानिवाताश्रयणे प्रयोजयेत् ॥

same śucau śarkarāvahnivālukā- vivarjite śabdajalāśrayādibhiḥ ।
mano’nukūle na tu cakṣupīḍane guhānivātāśrayaṇe prayojayet ॥

Let him perform his exercises in a place level, pure, free from pebbles, fire, and dust, delightful by its sounds, its water, and bowers, not painful to the eye, and full of shelters and caves.