Strike Him With a Piece of It

Why did Allah (swt) tell them to slaughter a cow? Because Allah (swt) wanted to perform an ayah, a miracle, in front of their eyes.

And remember when you killed a man and fell into dispute among yourselves as to the crime, but Allah brought forth that which you were hiding. So We said: strike him — the dead man — with a piece of the cow. Take a piece of that cow, which is dead, and strike the dead man with it, and he is going to have life. So the dead man who had been killed — when they touched him with the dead flesh of the cow, the man stood up alive, and he started to speak, and he said: my nephew is the one who killed me. Because he had killed him for the inheritance. My nephew is the one who killed me — and then he died again. It was a miracle of Allah (swt). Allah says: thus Allah brings the dead to life, and shows you His ayat, His signs, so that you may understand.