From the Goblet of Fire:
Voldemort smiled his terrible smile, his red eyes blank and pitiless.
'Wormtail's body, of course, was ill-adapted for possession, as all assumed him dead, and would attract far too much attention if seen. However, he was the able-bodied servant I needed, and, poor wizard though he is, Wormtail was able to follow the instructions I gave him, which would return me to a rudimentary, weak body of my own, a body I would be able to inhabit while awaiting the essential ingredients for true rebirth ... a spell or two of my own invention ... a little help from my dear Nagini' - Voldemort's red eyes fell upon the continually circling snake - 'a potion concocted from unicorn blood, and the snake venom Nagini provided ... I was soon returned to an almost human form, and strong enough to travel. [...]'
Harry Potter and the Goblet of Fire - p.569 - Bloomsbury - Chapter 33, The Death Eaters