Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother, thank you for your question and for your concern to follow Islam’s rulings concerning in your financial matters.
Responding to your question, Prof. Dr. Monzer Kahf, a prominent Muslim economist and counselor, states,
Generally speaking, lying is not permissible, but declaring part of the income that is documented and not telling about all income is permissible in my opinion. This is a civil matter not a religious one (except for plain lying which is not permissible religiously). This applies to all countries in matters of all taxes.
Allah Almighty knows best.