The Alter Rebbe sent a copy of the Tanya to Reb Zusya, asking him to write an approbation. Reb Zusya, highly impressed with the manuscript said, "Of course, Zusya will write a recommendation for this holy text." So they brought him a pen and paper.

But Zusya said, "Is one piece of paper enough to write praises for a work like this? Zusya needs more!"

So they brought him another piece of paper.

But again he said, "It's not enough."

So they brought him more and more until all the paper in the house was before him.

But still he insisted, "It's not enough!"

So they brought him paper from the neighbors, so much that there was a mountainous pile on the table, reaching almost to the ceiling.

And he still wasn't satisfied.

Finally he said, "Zusya now sees that there isn’t enough paper in the entire world to write an approbation for the Tanya. And if Zusya can't fully detail the praises that this holy work deserves, even with all the paper in the world, then Zusya will just use the first piece of paper that you brought to him."

So Reb Zusya used the original piece of paper to pen his letter of recommendation, and that is the approbation printed at the beginning of the Tanya.

Source: Beit Rebbi p. 156