When you know you don’t have time for a task a colleague is asking you to do, you can simply directly say, “I’m sorry, I’m unable to help you with this right now,” and negotiate to complete the request at a later time or with a different scope, or refer them to a different colleague who could help, says Mary Abbajay, president of the leadership development consultancy Careerstone Group.
Article source: https://www.huffingtonpost.co.uk/entry/how-to-say-no-at-work-professionally_uk_628e6947e4b0cda85db97454