Since \(n\) is composite then:
$$n=ab$$
Assume \(a \le b\), then \(a \le \sqrt{n}\), otherwise:
$$\begin{align} a \gt \sqrt{n} &\implies b \gt \sqrt{n} \\ &\implies ab \gt \sqrt{n}\sqrt{n} \\ &\implies ab \gt n \end{align}$$
According to the Fundamental Theorem of Arithmetic, there is a prime \(p\) such that \(p|a\). That means \(p \le \sqrt{n} \).