Śatapatha Brāhmaṇa ("of a hundred paths") 11.4.2.3

11.4.2.3

athāto'tikramaṇasya vajreṇa ha vā anyo'dhvaryuryajamānasya paśūnvidhamati vajreṇa hāsmā anya upasamūhatyeṣa ha vā adhvaryurvajreṇa yajamānasya paśūnvidhamati ya āśrāvayiṣyandakṣiṇenātikrāmati savyenāśrāvyātha hāsmā eṣa upasamūhati ya āśrāvayiṣyantsavyenātikrāmati dakṣiṇenāśrāvyaiṣa hāsmā upasamūhati

Now as to the stepping past (the Vedi). By a thunderbolt, indeed, one, Adhvaryu scatters the Sacrificer's cattle, and by a thunderbolt another drives them together for him. Now that Adhvaryu, doubtless, scatters the Sacrificer's cattle by a thunderbolt, who steps past with his right (foot) when he is about to call (on the Âgnîdhra) for the Sraushat, and with his left (foot) when he has called for the Sraushat; and that (other) one drives them together for him who steps past with his left (foot) When he is about to call for the Sraushat, and with his right (foot) when he has called for the Sraushat; for he does indeed drive them together for him.