Musnad Ahmad 1012:35

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, 'Ubadah ibn al-Walid ibn 'Ubadah ibn al-Samit narrated to me, from his father al-Walid, from his grandfather 'Ubadah ibn al-Samit—and he was one of the Nudaba' (chieftains)—who said: "We pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, the pledge of war." And 'Ubadah was one of the twelve who pledged allegiance at the first 'Aqabah upon the pledge of the women: to listen and obey in our hardship and our ease, and in our willingness and our unwillingness, and that we shall not dispute the command with those who hold it, and that we shall speak the truth wherever we are, not fearing for the sake of Allah the blame of any blamer.