I just added a line to my signiture that I found on the Cat PopurrE Website. Is it necessary to indicate the source of this line or can it be left out?

It's always nice to give credit. There are copyright laws but I doubt someone is going to sue you over using a quote without credit here at the Cat Site. But I think it's nice to give credit where credit is due. I would hope that if someone were quoting me they would give me credit.