ANSWER: IT'S CALLED THE ZEROTH LAW OF THERMODYNAMICS
EXPLANATION:
The zeroth law of thermodynamics state that there's no heat transferred between two objects in thermal equilibrium (i.e having the same temperature). This phenomenon depicts that there's balance in the amount of temperature in all the three entities.
This phenomenon was credited to Sir Ralph Howard Fowler, a British physicist and astronomer during his lifetime.
A scientific paper by Carathéodory also added that when two systems S1 and S2 are made to reach equilibrium with a third system S3 under close thermal conditions, systems S1 and S2 are said to be in mutual equilibrium.