Proof: By Induction
(related to Proposition: Existence of Solutions of an LDE With More Variables)
By hypothesis, $m > 0$ is a given positive integer $a_1,\ldots,a_r,b$ are given integers.
"$\Rightarrow$"
 Assume, the LDE (linear Diophantine equation) with $r$ variables $a_1x_1+\ldots+a_rx_r=b$ is solvable.
"$\Leftarrow$"
 Now, assume that $d\mid b.$
 Case $r=1$: All except one coefficient $a_i$ equal zero, without loss of generality $a_1\neq 0$ i.e. $a_1x_1+0x_2+\ldots+0x_r=b.$
* Then, obviously, $d=\gcd(a_1,0,\ldots,0)=a_i\mid b$ and we have that the equation is solvable.
 Case $r=2$: All except two coefficients $a_i$ equal zero, without loss of generality $a_1\neq 0,a_2\neq 0$ i.e. $a_1x_1+a_2x_2+0x_3+\ldots+0x_r=b.$
* Note that from the existence and number of solutions of an LDE with one variable, it follows that the linear Diophantine equation of congrences $(a_1x_1)(a_2)\equiv b(a_2)$ is solvable for the module $a_2$ if and only if $\gcd(a_1,a_2)\mid b.$
* With this solution, we can also solve^{1} the above equation $a_1x_1+a_2x_2=b.$
 Case $r > 2$:
* Assume, the claim is correct for the cases $2,\ldots,r1$ (base case). By induction:
* Set $a:=\gcd(a_1,\ldots,a_{r1}),$ then we have $\gcd(a,a_r)=d$ by definition of the greatest common divisor of more than two numbers.
* According to the case $r=2,$ the equation $ax+a_rx_r=b$ is solvable, for some suitable integers $x, x_r.$
* Since $a\mid ax$ we have also that the equation $a_1x_1+\ldots+a_{r1}x_{r1}=ax$ is solvable, for some suitable integers $x_1,\ldots,x_{r1},$ by the base case.
* Therefore, the equation $a_1x_1+\ldots+a_rx_r=b$ is solvable for some suitable integers $x_1,\ldots,x_{r}.$
∎
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References
Bibliography
 Landau, Edmund: "Vorlesungen über Zahlentheorie, Aus der Elementaren Zahlentheorie", S. Hirzel, Leipzig, 1927
Footnotes