What does one say if asked to perform a supererogatory prayer or voluntary fasting, and he does not wish to do so?

Allah Almighty has not obligated His servants to anything but the five daily prayers and fasting the month of Ramadan. And whoever commands something that Allah Almighty has not made obligatory, then obedience to him is not obligatory. If someone commands you to observe a supererogatory prayer or a supererogatory fast, it is a command to a recommended good deed, and you may pray for him and thank him for his advice. If you find the energy to do what he guided you to, then it is good; otherwise, there is no blame upon you for leaving recommended acts.