When the equivalent relaton $x\sim y \iff f(x)=f(y)$ gives a foliation?

Question

Let $M$ be an $n$-dimensional smooth manifold, and let $f:M\rightarrow R^m$ be a smooth map with constant rank $m$ with $0<m<n$. Now consider the equivalent relation $\forall x,y\in M \ x\sim y \iff f(x)=f(y)$. I'm interested in knowing under which conditions on the map $f$ the defined equivalent relation gives a foliation of M.