Abd al-Kabir al-Ghifari (may Allah have mercy on him) said: Udaysah bint Uhban bin Sayfi (may Allah have mercy on her) told me that Sayyiduna Ali (may Allah be pleased with him) came to her father, stood at the door, asked permission, and said: O Abu Muslim, what prevents you from striving in this matter and taking your share of it? He said: What prevents me from that is a covenant that my close friend and your cousin (peace and blessings be upon him) took from me; he (peace and blessings be upon him) commanded me that if tribulation breaks out, I should take up a sword of wood. So here it is with me; if you wish, I will fight with it.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 863