There are so many places in which Telemachus and Penelope show faithfulness. In light of this, I will give you a few examples.
In books 20-22, Telemachus and Odysseus get ready to attack the suitors and in book 22, they put their plans into action. Telemachus risks his own life. This fact alone shows that he is faithful. However, his faithfulness is seen even earlier. Remember that he makes a voyage to Pylos to see Nestor to ask about his father. The act took courage (book 3).
When it comes to Penelope, her faithfulness is deep. For many years (20 years) she warded off the suitors. She even concocted a plan to ward them off. She stated that she would marry someone only after she finished a funeral robe for her father-in-law. During the day she would weave and at night she would unweave. Here is what Homer says concerning this:
"So she said, and though proud we agreed. Then day after day she wove the great web, but at night, by torchlight, she unmade it. So for three years she cunningly kept the Achaeans from knowing, and so tricked them." 2.110
In addition, when Penelope finally sees Odysseus in book 23 she does not quite believe. So, she tests him. It is only when she is persuaded that she give herself to him.