Ganesha's tusk and Mahabharata dictation

Puzzle: Where the story of Ganesh using his tusk as stylus to take Vyasa's dictation is to be found?

Dr. Prema Nandakumar has kindly provided the answer:

The information is given in the very first verse of Villi Bharatham and is quite famous as the "Kadavul Vaazhththu" (Prayer to the Divine). A rough/prosaic but literal translation would be:
"To make it firm as the fifth along with the four Vedas
On this sea-girt earth,
When the Mahabharata was spoken by the King of sages
Who possess unfading askesis and Truth,
Vinayaka wrote with the Mount Meru
As the cadjan leaf,
And His own tusk as the sharp stylus.
I salute Him with shoreless love."

(fifth along with the four Vedas -- Bharatah Panchamao Vedah; Vyasa is described as the king of sages; the sages are described as possessing unfading askesis and Truth (vaadaa thavam meymmai muni raajan); Mount Meru is described as "edu" which is palm or cadjan leaf).

Some copies of the work tack this and the following verse which is also a prayer to Vinayaka to the verses of praise by Villi's son Varantharuvaar that are always printed as a preface to the work.