Is there a FAR/DFAR reference that support the SOW is required in a government task order
FAR 16.505 covers placing orders under indefinite-delivery contracts. Per Paragraph (a)(2), Individual orders shall clearly describe all services to be performed or supplies to be delivered so the full cost or price for the performance of the work can be established when the order is placed. Orders shall be within the scope, issued within the period of performance, and be within the maximum value of the contract.
That being said, if the orders are within scope, there is no need to need to attach the SOW to the Task Order.
Additionally, per Paragraph (a)(7) of the same citation, Orders placed under indefinite-delivery contracts must contain the following information:
(i) Date of order.
(ii) Contract number and order number.
(iii) For supplies and services, line item number, subline item number (if applicable), description, quantity, and unit price or estimated cost and fee (as applicable). The corresponding line item number and subline item number from the base contract shall also be included.
(iv) Delivery or performance schedule.
(v) Place of delivery or performance (including consignee).
(vi) Any packaging, packing, and shipping instructions.
(vii) Accounting and appropriation data.
(viii) Method of payment and payment office, if not specified in the contract (see 32.1110(e)).
(ix) North American Industry Classification System code (see 19.102(b)(3)).
Even though the Statement of Work for the basic contract applies for all Task/Delivery orders under indefinite-delivery contracts, there is nothing in the FAR that states you can't attach the SOW to the Task Order.
It is usually safe to assume that you are allowed to perform an action unless the FAR specifically states that you cannot.