Will a person be held accountable for involuntary thoughts that occur to him during prayer and in Ramadan, knowing that the devils are chained, and will he be considered sinful because of them?

A servant is not held accountable for the thoughts and whispers that come to the soul as long as they do not go beyond mere self-talk to a firm intention to act. This is based on the Prophet Muhammad's (peace be upon him) saying: "Indeed, Allah has forgiven my Ummah for what they self-talk about, as long as they do not act upon it or speak about it."