Manava Dharmashastra 6.24

6.24

upaspṛśaṃs triṣavaṇaṃ pitṝn devāṃś ca tarpayet
tapaś caraṃś cogrataraṃ śoṣayed deham ātmanaḥ //

When he bathes at the three Savanas (sunrise, midday, and sunset), let him offer libations of water to the manes and the gods, and practising harsher and harsher austerities, let him dry up his bodily frame.