TL;DR: No such words were spoken by Gaṇeśa in the Mahābhārata i.e., you cannot find a reference to the original Sanskrit verse for the quote you mention in the question.
The entire conversation between Vyāsa and Gaṇeśa from the very first chapter of the Mahābhārata is as follows:
काव्यस्य लेखानार्थाय गणेश: स्मर्यतां मुने।
सौतिरुवाच एवमाभाष्य तं ब्रह्मा जगाम स्वं निवेशनम्॥७४॥
Let Ganesha be remembered, O Rishi, to write this poem. Sauti said:
Having thus spoken to Vyasa, Brahma went away to his own place.
ततः सस्मार हेरम्बं व्यासः सत्यवतीसुतः। स्मृतमात्रो गणेशानो भक्तचिन्तितपूरकः ॥७५॥
तत्राजगाम विघ्नेशो वेदव्यासो यतः स्थितः। पूजितश्चोपविष्टश्च व्यासेनोक्तस्तदाऽनघ ॥७६॥
Then Vyasa remembered Ganesha in his mind. As soon as he was thought of the expeller of obstacles, Ganesha, who is always ready to fulfill the desire of his worshipers, came at once to the place where Vyasa was seated.
लेखको भारतस्यास्य भव त्वं गणनायक। मयैव प्रोच्यमानस्य मनसा कल्पितस्य च ॥७७॥
When he was saluted and when he took his seat, Vyasa thus addressed him, "O guide of the Ganas, kindly become the writer of the Bharata, which I have composed in my mind, but which I shall now repeat.'
श्रुत्वैतत्प्राह विघ्नेशो यदि मे लेखनी क्षणम्। लिखितो नावतिष्ठेत तदा स्यां लेखको ह्यहम् ॥७८॥
On hearing this Ganesha thus replied; I shall become the writer of your work, provided my pen is not made to stop even for a moment.
व्यासोऽप्युवाच तं देवमबुद्ध्वा मा लिख क्वचित्। ओमित्युक्त्वा गणेशोऽपि बभूव किल लेखकः ॥७९॥
Vyasa also told him, "without understanding it, please do not write anything.
Ganesha assented by saying “Om.” He proceeded to Write and Vyasa began to dictate.
ग्रन्थग्रन्थि तदा चक्रे मुनिगूढं कुतूहलात्। यस्मिन्प्रतिज्ञया प्राह मुनिद्वैपायनस्त्विदम् ॥८०॥
To take time to rest, Vyasa Sometimes knit the knots of composition very close. Thus he went on dictating his work as he made engagement.
Now it's possible that William Buck has picked up a version of the Mahābhārata, which is very different from what most Hindus or Indians know, as reference for his retelling of the Mahābhārata.
Here's how he narrates, in its entirety, the Vyāsa-Gaṇeśa talk:
Shiva's son came and asked, "Why call me?"
Vyasa replied, "Do you not remove all obstacles and barriers? You are the god of thieves and writers. Write down my book as I tell it to you."
Ganesha swished his trunk around. "OM! But there are books and books. Is yours a very good one?"
Ganesha laughed, and his huge belly shook. "Well just let me get rid of all these things..." He set down the conch
shell and lotus, the discus and axe that he held in his four hands. " ... and I shall write for you; but if once you stop the story, I will leave and never return."
Vyasa said, "On this condition: if you don't understand what I mean, you must write no more until you do."
"Done! The very day I was born I made my first mistake, and by that path have I sought wisdom ever since."
I was born fullgrown from the dew of my mother's body. We were alone, and Devi told me, "Guard the door. Let no one enter, because I'm going to take a bath." Then Shiva, whom I had never seen, came home. I would not let him into his own house.
"Who are you to stop me?" he raged.
And I told him, "No beggars here, so go away!"
"I may be half naked," he answered, "but all the world is mine, though I care not for it."
"Then go drag about your world, but not Parvati's mountain homel I am Shiva's son and guard this door for her with my life!"
"Well," he said, "you are a great liar. Do you think I don't know my own sons?"
"Foolishness!" I said. "I was only born today, but I know a rag picker when I see one. Now get on your way."
He fixed his eyes on me and very calmly asked, "Will you let me in?"
"Ask no more!" I said. "Then I shall not," he replied, and with a sharp glance he cut off my head and threw it far away, beyond the Himalayas. Devi ran out, crying, "You'll never amount to anything You've killed our son!" She bent over my body and wept.
"What good are you for a husband? You wander away and leave me home to do all the work. Because you wander around dreaming all the time, we have to live in poverty with hardly enough to eat."
The Lord of All the Worlds pacified her; looking around, the first head he saw happened to be an elephant's, and he set it on my shoulders and restored me to life.
"Parvati was happy again, and that is how I first met my father," said Ganesha, "long, long ago."
"Alright," said Vyasa, "now I will begin." And he began to tell his story to Ganesha, who wrote it on leaves.
"And many times Vyasa would compose lines that would make Ganesha pause, so he could use the time to think out the next part," said Sauti.
Also, interestingly, from this Amazon.com review, it appears that William Buck's retelling of Mahābhārata is very different from the traditional account of the story:
Many readers, including me, appreciate the artistry and skill of Buck's writing. I am puzzled, however, by the changes that he made in the story. For example, Buck has Krishna kill Dushsasana prior to the battle (in Vyasa, Dushsasana is killed by Bhima at Kurukshetra). For another example, in Buck's version, it is Virata's son Uttara who breaks into the Kuru's Lotus formation [padma-vyūha] during the battle (and is subsequently killed). In Vyasa, the protagonist is Abhimanyu, son of Arjuna. For a third example, Buck has Draupadi volunteer, after having been won by Arjuna, to become the wife of the other four Pandava brothers as well; in Vyasa she has no choice, since her mother-in-law, Kunti, commanded Arjuna to share "whatever he had brought" with his brothers.
By Michael Gunther on July 3, 2003