Skandapurana 5.161.42

5.161.42

pākhaṃḍaiḥ patitaiḥ sārdhaṃ tasminnahani nālapet ..
bhakṣayitvā nṛpaśreṣṭha prabhāte dantadhāvanam ..
maṃtreṇānena paścācca kartavyo niyamo nṛpa .. 42 ..

On that day one should not converse with heretics or fallen persons. / Having cleaned one's teeth at dawn, O best of kings, / a vow is then to be made with this mantra, O king: