You have to read book 3, HARRY POTTER AND THE PRISONER OF AZKABAN.

When everyone is leaving the passage on page 379-380 (chapter 20) it states that "Crookshanks darted up first; he had evidently pressed his paw to the knot on the trunk, because Lupin, Pettigrew, and Ron clambered upward without any sound of savaging branches. Black saw Snape up through the hole, then stood back for Harry and Hermione to pass. At last, all of them were out."

Then, in chapter 21, after Harry and Hermione turn back time and save Buckbeak and wait for themselves to leave the passage, it states

"'Here we come!' Hermione whispered. She and Harry got to their feet. Buckbeak raised his head. They saw Lupin, Ron, and Pettigrew clambering awkwardly out of the hole in the roots. The came Hermion...then the unconscious Snape, drifting weirdly upward. Next came Harry and Black. They all began to walk toward the castle."

Just wanted to point this out! Have fun reading the books!