A good place to start to find quotes regarding Ophelia's madness is clearly Act IV scene 5, when Ophelia is presented as being mad for the first time. Note how Horatio describes Ophelia to Gertrude, who is adamant that she will not see her:
She is importunate, indeed distract,
Her mood will needs be pitied.
The madness of Ophelia is clearly indicated through her conversation with Gertrude and the kind of nonsense she produces, giving testament to the deep level of mental suffering she is experiencing. Note the following example:
They say the owl was a baker's daughter. Lord, we know what we are, but know not what we may be. God be at your table.
The way that ludicrous speech is interspersed with serious sayings and her lack of formality and regard for the King and Queen of Denmark clearly indicate the extent of her madness.
Lastly, note the response of Laertes to seeing his sister:
O Heavens, is't possible a young maid's wits
Should be as mortal as an old man's life?
This quote clearly links Ophelia's madness to its cause: the murder of their father, Polonius, which shows the strength of Ophelia's emotion for her father, and, of course, for Hamlet, who killed him.