Rigvidhana 1.7.33

1.7.33

tryaham prātas tryaham sāyam tryaham adyād ayācitam./
tryaham param ca na aśnīyāt prājāpatyam caran dvijaḥ.//

For three days in the morning, for three days in the evening, for three days he should eat unasked food, / and for three days further he should not eat: [thus] a twice-born performs the Prājāpatya penance.