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;
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".
The quotation, which may not in fact be a quotation from the Scripture, could have come from a non-biblical source that was familiar to à Kempis.
Otherwise, it suggests to me a conflation of several biblical concepts such as we find in Matthew 5:4, 6:6, and 6:12.
"Blessed are those who mourn, for they shall be comforted."
"But you, when you pray, go into your inner room, close your door and pray to your Father who is in secret, and your Father who sees what is done in secret will reward you."
"And forgive us our debts, as we also have forgiven our debtors."
Put them all together, and you have the substance of à Kempis' quotation. I suggest the following paraphrase of the quotation:
"If you want to experience true heart-sorrow, go into your private prayer closet where only God can see and hear you and where the din of the outside world is silenced; repent of and mourn for your sins; and receive the gift of forgiveness in proportion to your willingness to forgive others who may have sinned against you."
Wow! Nice detective work, Paul A. Clayton. To add to your excellent answer... The original Latin in De Imitatione Christi is: Si vis cordetenus compungi; intra cubile tuum et exclude tumultus mundi: sicut scriptum est, In cubilibus vestris compungimini.
Thomas à Kempis is quoting the (old) Vulgate (not the corrected Nova Vulgata), which reads for Psalms 4:5 — quæ dicitis in cordibus vestris, in cubilibus vestris compungimini, literally: "[those things] which you say in your hearts, repent [for them] in your chambers".
The Masoretic Hebrew text for this verse reads אִמְרוּ בִלְבַבְכֶם עַל-מִשְׁכַּבְכֶם וְדֹמּוּ, imru bilevavkhem 'al mishkavkhem wedommu, "speak to your hearts on your beds and be silent", while it seems that Jerome was working with a different Hebrew text (noted in my BHS as a variant reading in the Greek manuscripts) where the conduction 'we' [and] was moved two words up: *imru bilevavkhem we'al mishkavkhem dommu, "speak to your hearts, and on your beds be silent."
