Is it true that a person should not guarantee the acceptance of their prayer?

Prayer is a great pillar of Islam, and one must be diligent in performing it with its conditions, pillars, duties, and recommended acts, alongside humility and presence of heart, and avoiding what invalidates it. Nevertheless, its acceptance cannot be guaranteed, for acceptance is from the grace and success granted by Allah. Indeed, entering Paradise is not by deeds but by the grace and mercy of Allah, as the Prophet, peace and blessings be upon him, said: "No one's deeds will ever admit him to Paradise... except that Allah envelops me in His grace and mercy." And among the characteristics of the pious is their fear that Allah may not accept their acts of obedience from them.