My office deducts tax on overtime payment , please I need your advise whether or not it should be taxed if not, I need valid/documented proof to fight this e.g. section in Nigeria Labour Law or federal constitution,

Your office is right. It is not a matter of labour laws or federal constitution. The law that guide your taxes as an employee is Personal Income Tax Act (PITA) as applicable in your place of work/principal residence. PITA requires that all incomes including 'upfront payments' are taxable subject to some claimable reliefs. The only receipt from your office that is not taxable is reimbursement of expenses that you specifically incurred eg travelling expenses but note that your transport allowance that is included in your salary will be taxed