Musnad Ahmad 8:70

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Qatadah, from 'Ikrimah, from Ibn Sa'd, from Sa'd, from the Prophet, may Allah bless him and grant him peace, that he said regarding the plague: "If it occurs in a land, do not enter it, and if you are in it, do not flee from it." Shu'bah said: And Hisham Abu Bakr narrated to me that it is 'Ikrimah ibn Khalid.