According to Newton’s second law of motion, the net external force acting on a body or system is equal to the rate of change of its momentum.
i.e. Fext=ΔP/Δt
Fext = Net external force acting on the system
P = Momentum of the system
When Fext=0, ΔPΔt=0
⇒ΔP = 0
⇒P = constant
Therefore momentum of the system will be conserved when no net external force acts on the system.