Mahabharata 13.107.142

पत्नीं रजस्वलां चैव नाभिगच्छेन्न चाह्वयेत्स्नातां चतुर्थे दिवसे रात्रौ गच्छेद्विचक्षणः

patnīṃ rajasvalāṃ caiva nābhigacchenna cāhvayetsnātāṃ caturthe divase rātrau gacchedvicakṣaṇaḥ

Our plain-language meaning of this verse is waiting for scholar review.