If in a nuclear fusion process the masses of the fusing nuclei be m₁ and m₂ and the mass of the resultant nucleus be m₃, then

$m_3<\left(m_1+m_2\right)$

In nuclear fusion, the mass of end product/resultant is always less than the sum of the initial product, the rest is liberated in the form of energy like in sun, energy is liberated due to the fusion of $2$ hydrogen atoms.

m₃​<(m_1​+m₂​)