In the battle of Siffin Amir al-mu'minin saw Imam al-Hasan proceeding rapidly to fight, then he said:
Hold back this young man on my behalf, lest he causes my ruin, because I am loath to send these two (meaning al-Hasan and al-Husayn) towards death, lest the descending line of the Prophet - may Allah bless him and his descendants - is cut away by their death.
(1) Al-Tabari, Ta'rikh, VI, 34, events of 37 H.; V, 196, events of 36 H.