HP&POA pg. 380 (paperback, US version):
Harry could see Lupin's silhouette. he had gone rigid. Then his limbs began to shake.
"Oh, my --" Hermione gasped. "He didn't take his potion tonight! He's not safe!"
"Run," Black whispered. "Run. Now."
But Harry couldn't run. Ron was chained to Pettigrew and Lupin. He leapt forward but Black caught him around the chest and threw him back.
"Leave it to me - RUN!"
There was a terrible snarling noise. Lupin's head was lenghthening. So was his body. His shoulders were hunching. Hair was sprouting visibly on his face and hands, which were curling into clawed paws. Crookshanks's hair was on end atain; he was backing away --
As the werewolf reared, snapping its long jaws, Sirius disappeared from Harry's side. He had transformed. The enormous bearlike dog bounded forward.