Musannaf Ibn Abi Shayba 28:694

Yahya ibn Zakariyya ibn Abi Za'idah narrated to us, from Mujalid, from 'Amir, who said: 'Umar wrote to Sa'd on the day of al-Qadisiyyah: "I have sent to you the people of the Hijaz and the people of Syria. So whoever among them reaches the fighting before they have dispersed, then assign a share [of the spoils] to them."