This Hadith is in light of  Superiority of forth Caliph of Muslim Hazrat Ali (R.A). The reference is taken from Imam Abu Hanifa's book Musnad Imam-ul-Azam Abu Hanifa. You can find other Hadiths on Superiority of Hazrat Ali (R.A) from the book Tareekh ul Khulafa  by Imam Souyouti in my earlier post