When two light nuclei (A<20) X and Y combine to form more tightly bound bigger nucleus Z, the process is nuclear fusion $\mathrm{x}+\mathrm{y}\to \mathrm{z}$ In the figure shown is BE/A versus ‘A’ curve (All terms have their usual meaning)  

Element Z is more stable than elements X and Y. So total binding energy of Z is more than sum of the binding energies of X and Y . Therefore a fraction of the total rest mass of X and Y is converted into binding energy of z . So $△\mathrm{m} =(\mathrm{Total} \mathrm{initial} \mathrm{rest} \mathrm{mass} - \mathrm{total} \mathrm{final} \mathrm{rest} \mathrm{mass} ) \mathrm{is} \mathrm{positive} .$

Hence Q = $△m\times c^2 is positive and energy is released in the process.$