Manava Dharmashastra 3.208

3.208

āsaneṣūpakḷpteṣu barhiṣmatsu pṛthakpṛthak
upaspṛṣṭodakān samyag viprāṃs tān upaveśayet //

The (sacrificer) shall make the (invited) Brahmanas, who have duly performed their ablutions, sit down on separate, prepared seats, on which blades of Kusa grass have been placed.