The purpose of this talk is to derive a rigidity theorem à la Eichmayr-Galloway-Mendes (DOI:10.1007/s00220-021-04033-x) in the spin setting. The statement is concerned with initial data sets $(g,k)$ on a manifold $M$ with boundary such that $(g,k)$ satisfies the dominant energy condition as well as a condition for the null expansion scalars along the boundary. Using Dirac-Witten operators we prove that $M$ must be diffeomorphic to a cylinder $N \times [0,1]$ and is foliated by MOTS carrying non-trivial parallel spinors for the induced metrics. As a special case we also obtain a rigidity statement for Riemannian metrics of non-negative scalar curvature and with mean convex boundary.