Uttarajjhayana 211.22.p1.36

211.22.p1.36

sayaṇāsaṇaṭhāṇe vā | je u bhikkhū na vāvare /
kāyassa viussaggo | chaṭho so parikittio ||36||

If a monk remains motionless when lying down, sitting, or standing upright, this is called abandoning of the body, which is the sixth kind (of internal austerities). (36)
If a sage truly performs these two kinds of austerities, he will soon be thoroughly released from the Circle of Births. (37)
Thus I say.