What is the source of the phrase "in your chamber bewail your sins"? In the Imitation of Christ, Thomas à Kempis writes (Book I, Chapter XX, paragraph 5):


  If you desire true sorrow of heart, seek the privacy of your cell and
  shut out the uproar of the world, as it is written: “In your chamber
  bewail your sins.”


Does “In your chamber bewail your sins” refer to a particular Scripture passage, or is something else in mind?
The Rev. William Benham's translation (available at Project Gutenberg) references this as Psalm 4:4. That translation renders the sentence thus:


  If thou wilt feel compunction within thy heart, enter into thy chamber and shut out the tumults of the world, as it is written, Commune with your own heart in your own chamber and be still.


The ESV translates Psalm 4:4 thus:


  Be angry, and do not sin;
  &nbsp;&nbsp;&nbsp;&nbsp;ponder in your own hearts on your beds, and be silent.


The Douay-Rheims 1899 American Edition might be closer to the Vulgate used by Thomas' community since the Septuagint on which it is based might vary a bit from the Hebrew texts preferred by the ESV translators. It translates the verse (4:5, because the annotation text is treated as verse 1): 


  Be angry, and sin not: the things you say in your hearts, be sorry for them upon your beds.


"be sorry for them upon your beds" is reasonably close to "in your chamber bewail your sins".