Musannaf Ibn Abi Shayba 38:26

Ibn ‘Ulayyah narrated to us, from Mansur ibn ‘Abd al-Rahman, from al-Sha’bi, who said: None of the Companions of the Prophet, may Allah bless him and grant him peace, from the Emigrants (Muhajirun) and the Helpers (Ansar) witnessed the Battle of the Camel except for ‘Ali, ‘Ammar, Talhah, and al-Zubayr. If they bring a fifth person, then I am a liar.