There are strict regulations on importing and exporting money. You can important up to $10,000 (or equivalent) and export $3,000 (or equivalent) without declaring it at customs.
If you export between $3,000 and $10,000 (or equivalent) it must be declared on departure. For anything imported, or exported, over the equivalent of $10,000 you'll need to provide proof that you've imported it into the country or legally obtained it whilst in the country.
If you complete a declaration form stating you are bringing in over $3,000, or equivalent, it needs to be stamped by a customs official or it will not be valid. Declaration forms are freely available at ports of entry.
If you obtain any money from a bank, make sure to keep all documents.
Be aware: If you fail to complete a declaration form on arrival or if you don't get it validated your foreign currency and non-declared items may be confiscated when you exit Russia and you can be fined.