Aśauca-vidhi (Rules of Impurity) within Śrāddha-kalpa — Chapter on Testing/Selecting Brahmanas and Honoring the Atithi
आमन्त्रितस्तु यः श्राद्धे योषितं सेवते द्विजः / पितरस्तस्य तन्मासं तस्मिन्रितसि शेरते
āmantritastu yaḥ śrāddhe yoṣitaṃ sevate dvijaḥ / pitarastasya tanmāsaṃ tasminritasi śerate
If a dvija, though invited to a śrāddha, indulges in union with a woman, his pitṛs lie prostrate for that whole month in that season.