Is there any canon reference of someone saying an incantation when apparating?
I don't remember one and even on the lessons in the sixth year, the teacher never mentions any.
There isn't a spell. We see the complete instructions, and it doesn't have a spell:
He waved his wand. Old-fashioned wooden hoops instantly appeared on the floor in from of every student.
“The important things to remember when Apparating are the three D’s!” said Twycross. “Destination, Determination, Deliberation!
“Step one: fix your mind firmly upon the desired destination,” said Twycross. “In this case, the interior of your hoop. Kindly concentrate upon that destination now.”
“Step two,” said Twycross, “focus your determination to occupy the visualised space! Let your yearning to enter it flood from your mind to every particle of your body! “
“Step three,” called Twycross, “only when I give the command... turn on the spot, feeling your way into nothingness, moving with deliberation. On my command, now... one —”
Harry Potter and the Half-Blood Prince, Chapter 18