It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Ali (may Allah be pleased with him) used to say during the lifetime of the Messenger of Allah (peace and blessings be upon him): "Indeed Allah, the Mighty and Majestic, says: 'If he dies or is killed, will you turn back on your heels?' [Al Imran: 144] By Allah! We will not turn back on our heels after Allah has guided us. By Allah! If he dies or is killed, I will keep fighting for that for which he fought until I die. By Allah! I am his brother, his friend, his cousin and his heir; so who has more right to him than I?"
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 175