One verse

Srimad Bhagavatam 11.18.4

11.18.4

ग्रीष्मे तप्येत पञ्चाग्नीन् वर्षास्वासारषाड्‍जले । आकण्ठमग्न: शिशिर एवंवृत्तस्तपश्चरेत् ॥ ४ ॥

grīṣme tapyeta pañcāgnīn varṣāsv āsāra-ṣāḍ jale ākaṇtha-magnaḥ śiśira evaṁ vṛttas tapaś caret

grīṣme
in the summer
tapyeta
one should accept as austerity
pañca-agnīn
five fires (the overhead sun and fires burning on four sides)
varṣāsu
during the rainy season
āsāra
torrents of rain
ṣāṭ
tolerating
jale
in water
ā-kaṇṭha
up to the neck
magnaḥ
submerged
śiśire
in the coldest part of winter
evam
thus
vṛttaḥ
engaged
tapaḥ
penance
caret
one should execute

Thus engaged as a vānaprastha, one should execute penance during the hottest summer days by subjecting oneself to burning fires on four sides and the blazing sun overhead; during the rainy season one should remain outside, subjecting oneself to torrents of rain; and in the freezing winter one should remain submerged in water up to one’s neck.