Civil Code Section 1950.5(g)(4)
The whole code should be read starting at 1950.5 for it to make sense. The best and easiest resource is the dca.ca.gov site which explains security deposit law and references each specific code.
Here's an excerpt from that site:
The landlord also must send you copies of receipts for the charges that the landlord incurred to repair or clean the rental unit and that the landlord deducted from your security deposit. The landlord must include the receipts with the itemized statement. The landlord must follow these rules:
If the landlord or the landlord's employees did the work - The itemized statement must describe the work performed, including the time spent and the hourly rate charged. The hourly rate must be reasonable.
If another person or business did the work - The landlord must provide you copies of the person's or business' invoice or receipt. The landlord must provide the person's or business' name, address and telephone number on the invoice or receipt, or in the itemized statement.
If the landlord deducted for materials or supplies - The landlord must provide you a copy of the invoice or receipt. If the item used to repair or clean the unit is something that the landlord purchases regularly or in bulk, the landlord must reasonably document the item's cost (for example, by an invoice, a receipt or a vendor's price list)
If the landlord made a good faith estimate of charges - The landlord is allowed to make a good faith estimate of charges and include the estimate in the itemized statement in two situations: (1) the repair is being done by the landlord or an employee and cannot reasonably be completed within the 21 days, or (2) services or materials are being supplied by another person or business and the landlord does not have the invoice or receipt within the 21 days. In either situation, the landlord may deduct the estimated amount from your security deposit. In situation (2), the landlord must include the name, address and telephone number of the person or business that is supplying the services or materials.
Within 14 calendar days after completing the repairs or receiving the invoice or receipt, the landlord must mail or deliver to you a correct itemized statement, the invoices and receipts described above, and any refund to which you are entitled.
The landlord must send the itemized statement, copies of invoices or receipts, and any good faith estimate to you at the address that you provide. If you do not provide an address, the landlord must send these documents to the address of the rental unit that you moved from.
The landlord is not required to send you copies of invoices or receipts, or a good faith estimate, if the repairs or cleaning cost less than $126 or if you waive your right to receive them