Musannaf Ibn Abi Shayba 3:5412

Waki' narrated to us, he said: Abu al-Minhal al-Ta'i Nasr ibn Aws narrated to us, from Abdullah ibn Zayd, from Abu Hurayrah, he said: "You must perform the two prostrations of the Duha (forenoon prayer), for they are better for you than two black-colored she-camels from the offspring of Banu Buhtur."