Proof
(related to Proposition: Multiplicativity of the Legendre Symbol)
 By hypothesis, $p > 2$ is a prime number and $r\ge 2$, $m,n_1,\ldots,n_r$ are integers.
 Obviously, if $p$ is a divisor of any of the integers, then, by definition, the corresponding Legendre symbol modulo $p$ equals $0.$
 Therefore, any product of Legendre symbols involving such an integers also trivially equals $0.$
 We shall therefore assume that $p$ does not divide any of the integers and look at the nontrivial cases:
 Case $r=2.$
 According to the Euler's criterion for quadratic residues, the Legendre symbol modulo $p$ of the product $nm$ fulfils the following congruence modulo $p$:
$$\left(\frac{nm}{p}\right)\equiv (nm)^{\frac {p1}2}\equiv (n)^{\frac {p1}2}\cdot (m)^{\frac {p1}2}\equiv \left(\frac{n}{p}\right)\cdot\left(\frac{m}{p}\right)\mod p.$$
 Now, observe that the difference $\left(\frac{nm}{p}\right)\left(\frac{n}{p}\right)\left(\frac{m}{p}\right)$ can only take one of the values $0,$ $2,$ or $2.$
 But since $p$ is odd, the above congruence holds if and only if $$\left(\frac{nm}{p}\right)\left(\frac{n}{p}\right)\left(\frac{m}{p}\right)=0.$$
 Case $r > 2.$
 This case follows by induction from the case $r=2.$
 Altogether, we have shown that the Legendre symbol modulo $p$ is a completely multiplicative arithmetic function.
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References
Bibliography
 Landau, Edmund: "Vorlesungen über Zahlentheorie, Aus der Elementaren Zahlentheorie", S. Hirzel, Leipzig, 1927