Musannaf Ibn Abi Shayba 27:869

Hashim ibn al-Qasim narrated to us, he said: Shu'bah narrated to us, from Khalid al-Hadhdha', from Ibn Sirin, that Abu Musa wrote to Umar regarding eighteen Bactrian camels that he had acquired. Umar wrote back to him: "Place them among the most courageous tribe of the Arabs." He said: So he placed them among Banu Rabah, a clan of Banu Tamim.