Musnad Ahmad 25:533

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Thawr ibn Zayd narrated to me, from 'Ikrimah, from Ibn 'Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) walked with them to Baqi' al-Gharqad, then he directed them and said: "Go forth in the name of Allah." And he said: "O Allah, help them"—meaning the group whom he directed to Ka'b ibn al-Ashraf.