“Heat cannot by itself flow from a body at lower temperature to a body at higher temperature” is a statement or consequence of

This is a consequence of second law of thermodynamics.

The Second Law of Thermodynamics has multiple formulations, but a common version can be summarized as follows:

"Natural processes tend to increase the overall entropy of a closed system and its surroundings."

Entropy is a measure of the amount of energy in a system that is unavailable to do useful work. In the context of your statement, if heat were allowed to flow from a colder body to a hotter body without any external influence, it would violate the second law because it would result in a decrease in the overall entropy of the system. Heat naturally flows from higher temperature regions to lower temperature regions, as this process leads to an increase in the overall entropy of the system and its surroundings.

The statement is an example of the principle that heat transfer occurs spontaneously in the direction that increases the overall entropy of the system. It also underscores the irreversibility of heat flow, meaning that once heat flows from a hotter body to a colder body, it cannot spontaneously flow back without some external intervention, such as the input of external energy (work).