Let $M$ be a magma with a binary operation $*_M$ and let $G$ be a group with a binary operation $*_G$.
If $f$ is a bijection from $M$ to $G$ preserving the operation, that is, $f(m_1 *_M m_2)=f(m_1)*_G f(m_2)$, then $(M, *_M)$ is a group.
I think this theorem is a powerful tool of proving a magma is a group. Does anyone know which books talk about this topic?