Natyashastra
व्याधिव्रीडितनिद्रासु ध्याने चोपविशेन्नरः । तथा चोत्कटिकं स्थानं स्फिक्पार्ष्णीनां समागमः ॥ २०८॥
In cases of illness, shame, sleep, or meditation, a person should sit accordingly. The utkaṭikā sitting posture consists of the hips and heels being brought together.
english translation
vyAdhivrIDitanidrAsu dhyAne copavizennaraH । tathA cotkaTikaM sthAnaM sphikpArSNInAM samAgamaH ॥ 208॥
hk transliteration by Sanscript