Vaishnava Dharmashastra 46.10

46.10

tryahaṃ sāyaṃ tryahaṃ prātas tryaham ayācitam aśnīyāt | eṣa prājāpatyaḥ ||

Let a man for three days eat in the evening only; for other three days, in the morning only; for further three days, food (given to him) unsolicited; (and let him fast entirely for three days): that is the Prâgâpatya (the penance invented by Pragâpati).