Al-Shafi (Shafi madhhab) was taught by Malik ibn Anas.
Ahmad ibn Hanbal (Hanbali madhhab) was taught by Abu Hanifa.
All 4 Sunni schools lead back to the teachings of Imam Jafar Sadiq a.s, who learnt from his grandfathers, Imam Hussain a.s, Imam Ali a.s and Prophet Muhammad ๏ทบ