John 8:56-59
56 Your father Abraham rejoiced to see my day: and he saw it, and was glad.
57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham?
58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
Jesus says, "Before Abraham was, I am", which is Jesus identifying Himself with Yahweh, referencing Exodus 3:14. The Jews take up stones to stone Him, because they recognize that this is a divine claim.
As covered in the discussion of John 8:24, Jesus saying "I am He" (egoĢ eimi) here refers to Him being the Messiah, and is not a claim to be the "I Am" of Exodus 3:14. This is how Jesus defines the phrase "I am He" when He first uses it in John's Gospel (John 4:25-26), and this is also the thesis of John's Gospel (John 20:30-31).
Here, in referencing Abraham, Jesus is placing Himself, the Messiah, above Abraham, and saying that Himself being the Messiah was a prophetic reality before Abraham was a physical reality, and that was why Abraham was able to "see (His) day" (John 8:56). The Jews attempt to stone Jesus, because they do not like Jesus, and do not think He is the Messiah, and so this claim enrages them.