Musannaf Ibn Abi Shayba 28:662

Waki’ narrated to us, he said: Muhammad ibn Rashid narrated from Sulayman ibn Musa, he said: Abu Musa wrote to Umar: "We, when we conquered Tustar, obtained broad-bodied horses." So he wrote to him: "Those are the packhorses (baradhin). Separate from them the noble ones (al-‘itaq), then distribute them as shares, and disregard what is other than that."