Musnad Ahmad 450:1

Yahya ibn Ishaq narrated to us, Yahya ibn Ayyub informed me, he said: Yazid ibn Abi Habib narrated to me, from Rabi'ah ibn Laqit, from Abdullah ibn Hawalah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever is saved from three, he has indeed been saved" — [he said it] three times: "My death, the Dajjal, and the killing of a Caliph who is patient with the truth, giving it [to those who deserve it]."