Musnad Ahmad 5:240

Hashim ibn al-Qasim narrated to us, Muhammad—meaning Ibn Rashid—narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Fadala ibn Abi Fadala al-Ansari—and Abu Fadala was one of the people of Badr—who said: I went out with my father to visit Ali ibn Abi Talib during an illness that had afflicted him, from which he had become heavy. He said: My father said to him, "What keeps you in this house of yours? If your appointed time were to come upon you, no one would attend to you except the Bedouins of Juhayna. Be carried to Medina, for if your appointed time comes upon you, your companions will attend to you and pray over you." Ali said: "The Messenger of Allah, may Allah bless him and grant him peace, made a covenant with me that I would not die until I am commanded, and then this—meaning his beard—will be dyed with the blood of this—meaning his head." He was then killed, and Abu Fadala was killed with Ali on the day of Siffin.