Skandapurana 17.15.41

17.15.41

ekabhaktena naktena tathaivāyācitena ca ..
upavāsevana kṛcchreṇa śākāhāreṇa vā punaḥ .. 41 ..

Or by [eating] a single meal, or at night, or [eating only] unasked-for food, / or by fasting, severe penance, or eating vegetables,