Mahabharata 13.92.13

पूर्वं पिण्डं पितुर्दद्यात्ततो दद्यात्पितामहेप्रपितामहाय च तत एष श्राद्धविधिः स्मृतः

pūrvaṃ piṇḍaṃ piturdadyāttato dadyātpitāmaheprapitāmahāya ca tata eṣa śrāddhavidhiḥ smṛtaḥ

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