One day Ibrahim (a.s) dreams that Allah gave him the order to sacrifice his most precious thing, his son Ismail (a.s) and Ibrahim (a.s) told this dream to Ismail (a.s) and Ismail (a.s) got ready to sacrifice himself. When Ibrahim (a.s) started to sacrifice his son Ismail (a.s) Allah replaced a sheep on the place of Ismail (a.s) and Ibrahim (a.s) sacrificed the sheep.
Every year Muslims from all over the world celebrate Eid-ul-Adha in the honour of Ibrahim (a.s) and follow the obedience of Allah.
Eid ul Azha is also called eid e qurban. All Muslims celebrate this eid by sacrificing animals like sheep, goat, camel and cow.