Manava Dharmashastra 3.216

3.216

nyupya piṇḍāṃs tatas tāṃs tu prayato vidhipūrvakam
teṣu darbheṣu taṃ hastaṃ nirmṛjyāl lepabhāginām //

Having offered those cakes according to the (prescribed) rule, being pure, let him wipe the same hand with (the roots of) those blades of Kusa grass for the sake of the (three ancestors) who partake of the wipings (lepa).