Musannaf Ibn Abi Shayba 36:104

Abu Usama narrated to us, he said: Isma'il narrated to us, from Qays, that the Prophet (peace and blessings of Allah be upon him) said to Bilal: "Have you prepared the riders or the group of the Bajilis?" He said: "No." He said: "Then prepare them, and begin with the Ahmasis before the Qasiris."