Manava Dharmashastra 3.279

3.279

prācīnāvītinā samyag apasavyam atandriṇā
pitryam ā nidhanāt kāryaṃ vidhivad darbhapāṇinā //

Let him, untired, duly perform the (rites) in honour of the manes in accordance with the prescribed rule, passing the sacred thread over the right shoulder, proceeding from the left to the right (and) holding Kusa grass in his hands, up to the end (of the ceremony).