Let $\mu$ be a $\sigma$-finite measure on a $\sigma$-Algebra $\mathcal A$ and $A_i\in\mathcal A$ ($i\in I$) subsets with $A_i\cap A_j=\emptyset$ for $i\neq j$. Then $\mu(A_i)>0$ for at most countably many $i\in I$.
I know $\sigma$-finite means that there exists a sequence $(A_n)\subset\mathcal A$ with $\Omega=\sum_{i=1}^\infty A_i$ such that $A_i\cap A_j=\emptyset$ for $i\neq j$ and $\mu(A_n)<\infty$ for all $n$. But I don't see how I can show the above using this.