Consider the following chemical equation for the combustion of methane (CH4)
Reactants have more energy than the products.
The substances reacting are methane and oxygen (reactants). They have some amount of energy inside them. When they react, products (carbon dioxide and water) are formed which also have some energy in them. When the total energy of the reactants is more than total energy of the products. This difference of energy is released as heat of the reaction.