Riether consulted fellow countryman and former Whites star Volz, who made 125 league appearances in five years at the Cottage.
Riether told the official website: "I played with Volz for the German national team at Under-21, Under-20 and Under-19 level. I spent a lot of time with him and he would often tell me about Fulham.
"When I told him Fulham were interested in me, he said it would be a good choice for me to come here.
"He said good things about the Club and in the past he would always speak positively about them and the mentality of the Premier League.
"I know the crowd at Fulham liked him. He is a fighter, he always gives 100 per cent and people like that. He is a good character, a good guy."