Musnad Ahmad 8:98

Bahz narrated to us, Shu'bah narrated to us, Habib ibn Abi Thabit informed me, he said: I arrived in Medina, and it reached us that the plague had occurred in Kufa. He said: So I said, "Who narrates this hadith?" It was said, "Amir ibn Sa'd." He said: And he was absent. So I met Ibrahim ibn Sa'd, and he narrated to me that he heard Usamah ibn Zayd narrating to Sa'd: That the Messenger of Allah, peace and blessings of Allah be upon him, said: "If the plague occurs in a land, do not enter it, and if it occurs while you are in it, do not exit from it." He said: I said, "Did you hear it from Usamah?" He said, "Yes."