Musnad Ahmad 1153:2333

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: And Salih ibn Kaysan narrated to me, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, from 'Aishah, she said: The last thing the Messenger of Allah, peace and blessings of Allah be upon him, enjoined was that he said: "Two religions shall not be left in the Arabian Peninsula."