Definite integral/Estimate for modulus/Exercise

Let ${}I=[a,b]$ be a compact interval and let ${}f\colon I\rightarrow \mathbb {R}$ be a Riemann-integrable function. Prove that

${}\vert {\int _{a}^{b}f(t)dt}\vert \leq \int _{a}^{b}\vert {f(t)}\vert dt\,.$

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