Amīr al-mu’minīn, peace be upon him, said: O’ al-Ḥārith! You have seen below yourself but not above yourself, and so you have been confused. Certainly, you have not known right, so that you can recognize the righteous. And you have not known wrong, so that you can recognize the people of wrong!
whereupon Amīr al-mu’minīn, peace be upon him, said: Verily, Sa‘d and ‘Umar have neither sided with Right nor forsaken Wrong. 
 Sa‘d ibn Mālik (i.e. Sa‘d ibn Abī Waqqāṣ, the father of ‘Umar ibn Sa’d who killed Imām Ḥusayn) and ‘Abdullāh ibn ‘Umar were among those who were keeping themselves away from Amīr al-mu’minīn’s help and support. As for Sa‘d ibn Abī Waqqās, after the killing of ‘Uthmān he retired to some wilderness and passed his life there, but did not agree to swear allegiance to Amīr al-mu’minīn (as Caliph). But after the death of Amīr al-mu’minīn he used to express his repentance, saying, “I held an opinion but it was a wrong opinion.” (al-Mustadrak, al-Hākim, vol. 3, p. 116). And when Mu‘āwiyah blamed him for not supporting him in his fight with Amīr al-mu’minīn, Sa‘d said :
I only repent for not having fought against the rebellious group (i.e. Mu‘āwiyah and his people). (Aḥkām al-Qur’ān, al-Jaṣṣāṣ al-Ḥanafī, vol. 2, pp. 224, 225; al-Furū‘, Ibn Mufliḥ al-Ḥanbalī, vol. 3, p. 542)
As for ‘Abdullāh ibn ‘Umar, although he had sworn allegiance, he refused to help Amīr al-mu’minīn in the battles putting forth the excuse: “I have sought seclusion to devote myself to worship and do not therefore want to involve myself in war and fighting.”
A Persian couplet says : Intelligence regards such excuses worse than the offence itself.
‘Abdullāh ibn ‘Umar also frequently used to express his repentance, even up to the last moments of his life, saying:
I do not find anything in myself to be distressed about in this world, except my not having fought alongside ‘Alī ibn Abī Ṭālib against the rebellious group as Allāh, to Whom belongs Might and Majesty, had commanded me. (al-Mustadrak, vol. 3, pp. 115-116; as-Sunan al-kubrā, al-Bayhaqī, vol. 8, p. 172; aṭ-Ṭabaqāt, Ibn Sa‘d, vol. 4, part 1, pp. 136, 137; al-Istī‘āb, vol. 3, p. 953; Usd al-ghābah, vol. 3, p. 229; vol. 4, p. 33; Majma‘ az-zawā’id, vol.3, p. 182; vol. 7, p. 242; al-Furū‘, vol. 3, p. 543; Rūḥ al-ma‘ānī, al-Alūsī, vol. 26, p. 151).