= Solution
Define asymptotic states $|\phi(k)\rangle=a_k^\dagger|0\rangle$ and $|\psi(p,s)\bar\psi(q,s')\rangle=b_{p,s}^\dagger c_{q,s'}^\dagger|0\rangle$. The <LSZ reduction formula> reduces the decay matrix element from the connected three-point function
$$
G(x,y,z)=\langle0|T\phi(x)\psi(y)\bar\psi(z)|0\rangle_c.
$$
In momentum space, amputate the scalar leg with $i(k^2-M^2)$ and the fermion legs with the inverse Dirac propagators, contract them with $\bar u^s(p)$ and $v^{s'}(q)$, and take $k^2=M^2$, $p^2=q^2=m^2$. The spacetime integrations produce $(2\pi)^4\delta^{(4)}(k-p-q)$.
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