Musnad Ahmad 32:1109

Abdullah said: I found in my father’s book, in his own handwriting: Zayd ibn al-Hubab narrated to us, he said: Amr ibn Hamzah informed me, Khalaf Abu al-Rabi’—the imam of the mosque of Sa’id ibn Abi Arubah—narrated to us, Anas ibn Malik narrated to us, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, this religion is firm, so penetrate into it with gentleness."