Manava Dharmashastra 9.186

9.186

trayāṇām udakaṃ kāryaṃ triṣu piṇḍaḥ pravartate
caturthaḥ saṃpradātaiṣāṃ pañcamo nopapadyate //

To three (ancestors) water must be offered, to three the funeral cake is given, the fourth (descendant is) the giver of these (oblations), the fifth has no connection (with them).