Shatapatha Brahmana 11.5.7.4

11.5.7.4

yadi ha vā apyabhyaktaḥ alaṃkṛtaḥ suhitaḥ sukhe śayane śayānaḥ svādhyāyamadhīta ā haiva sa nakhāgrebhyastapyate ya evaṃ vidvāntsvādhyāyamadhīte tasmātsvādhyāyo'dhyetavyaḥ

And, verily, if he studies his lesson, even though lying on a soft couch, anointed, adorned and completely satisfied, he is burned (with holy fire) up to the tips of his nails, whosoever, knowing this, studies his lesson: therefore one's (daily) lesson should be studied.