Ja'far al-Sadiq
Imam Ahmad ibn Hanbal, the founder of Hanbali school, was a disciple of the Sunni Imam Al-Shafi‘i, who was reportedly a student of Imam Malik ibn Anas, who was a student of the Imam Ja'far al-Sadiq, like Imam Abu Hanifa. Thus all of the four great Imams of Sunni "Fiqh" are connected to Imam Ja'far al-Sadiq from the "Bayt" (Household) of Muhammad, whether directly or indirectly.