Consider a sequence of two servers, when the output of the first one is the input of the second one.
The proof does iterative application of the definition of service curves X \ge A \otimes S_1, D \ge X \otimes S_2 and some properties of convolution, isotonicity (D \geq ( X \otimes S_2) \otimes S_1), and associativity (D \geq X \otimes (S_2 \otimes S_1)).