delta system lemma

If $S$ is a set of finite sets such that $|S|={\mathrm{\aleph }}_1$ then there is a $S^{\prime }\subseteq S$ such that $|S^{\prime }|={\mathrm{\aleph }}_1$ and $S$ is a http://planetmath.org/node/DeltaSystem$\mathrm{\Delta }$-system.