It was Eve. Genesis 3 tells the story of the Fall, and you can read this line in verse 13.
Above is how Titian painted the scene. Notice that the serpent's head looks like a sweet cherub!
This whole story shows that human beings are buck-passers. Adam and Eve both eat the apple that God has forbidden willingly. But when they get caught, Adam blames Eve and Eve blames the serpent. Not much has changed in the last 6,000 years!
