Which lines in act 3, scene 5 of Shakespeare’s Romeo and Juliet foreshadow the play's tragic ending?

The morning following their first night together as a married couple, Romeo and Juliet experience a mixture of exultation in one another's presence and a sense of foreboding about what the future might hold. When Juliet expresses her desire that Romeo not leave so quickly for Mantua, to escape capture for the murder of her cousin, he reminds her that if he delays, his death will be all but certain.
Sill pained by his imminent departure ("I must hear from thee every day in the hour, For in a minute there are many days . . . "), and well aware of the bitter enmity that separates their families, she wonders whether they will ever meet again. Although Romeo tries to assure her of future "sweet discourses," she foresees the worst.

"Oh God, I have an ill-divining soul!
Methinks I see thee, now thou art below,
As one dead in the bottom of a tomb:
Either my eyesight fails. or thou look'st
pale."

This is one of the tragic work's many passages of ominous foreshadowing. For it will not be long before Juliet does, indeed, behold her beloved in "the bottom of a tomb," before choosing to follow him in death.

In this scene, Romeo and Juliet bid each other farewell after spending their first night together as husband and wife in Juliet's bedchamber. Romeo has been banished to Mantua for killing Tybalt, and the audience knows this scene will be the last time they see each other alive (of course, Juliet is technically alive when Romeo encounters her lifeless body in the Capulet crypt, but he doesn't know that). As they reluctantly part, Romeo reassures his wife that they will see each other again soon, but Juliet fears otherwise, and tells her husband of a chilling vision:

O God, I have an ill-divining soul!Methinks I see thee, now thou art below,As one dead in the bottom of a tomb.

This exchange, like several others in the play, foreshadows the tragic end that the audience has known since the Prologue is imminent. As it turns out, Juliet's vision is sadly prophetic; she does, in fact, eventually find Romeo dead in a tomb. When Romeo encounters her body, he believes she is dead, and kills himself out of grief. Juliet awakes to discover him dead and takes her own life.