Shatapatha Brahmana 4.3.1.20

4.3.1.20

upayāmigṛhīto 'si | aṃhasaspataye tveti trayodaśaṃ grahaṃ gṛhṇāti yadi trayodaśaṃ gṛhṇīyādatha pratiprasthātādhvaryoḥ pātre saṃsravamavanayatyadhvaryurvā pratiprasthātuḥ pātre saṃsravamavanayatyāharati bhakṣam

With 'Thou art taken with a support: thee to Amhasaspati (lord of trouble)!' he (the Adhvaryu) draws the thirteenth libation, if he draw a thirteenth. The Pratiprasthâtri then pours his residue into the Adhvaryu's vessel, or the Adhvaryu pours his residue into the Pratiprasthâtri's vessel. He (the Adhvaryu) takes it (to the Sadas) for the purpose of drinking it.