Is humility (khushu') required in supererogatory (nafl) prayers and regular sunnah prayers, similar to the obligatory prayers for which a person will be held accountable on the Day of Judgment?

Reverence (khushu') is commanded in all prayers, and it is one of the most important reasons for increasing reward. Allah has praised the reverent, saying: "Successful indeed are the believers those who humble themselves in their prayers." The reward of the worshipper varies according to their reverence, as mentioned in the Prophet's (peace be upon him) saying: "Indeed, a servant may perform a prayer, yet nothing of it is recorded for him except a tenth of it..." Therefore, one should strive to perfect reverence and presence of heart in all prayers.