Aśauca-vidhi (Rules of Impurity) within Śrāddha-kalpa — Chapter on Testing/Selecting Brahmanas and Honoring the Atithi
यो दद्याच्चानुसंमोहान्न तद्गच्छति वै पितॄन् / तस्मान्न देयमन्नाद्यमुच्छिष्टं श्राद्धकर्मणि
yo dadyāccānusaṃmohānna tadgacchati vai pitṝn / tasmānna deyamannādyamucchiṣṭaṃ śrāddhakarmaṇi
If one, through delusion (unknowing), gives such food, it does not reach the Pitṛs. Therefore, in the śrāddha rite one should not give leavings (ucchiṣṭa) of food and the like.