What is the supplication one can make to restore sincerity in their heart, and what can be done if there is no specific supplication, for someone who feels the presence of hypocritical traits within themselves and lacks sincerity in their prayer?

Sincerity (Ikhlas) is of immense importance. Allah accepts only those deeds that are purely for His sake alone. If one associates partners with Him in a deed, He abandons that deed and the partnership. To achieve sincerity, one must: be mindful of the greatness of Allah, strive against the self (nafs), understand the danger of showing off (riya'), and seek help through supplication (du'a).

As for prayer (Salat), its status is عظيمة (great), and the punishment for abandoning it is painful. It will be the first thing a servant is held accountable for on the Day of Judgment. Negligence in prayer is a characteristic of the hypocrites, who will be in the lowest depths of the Hellfire. Feeling pain over one's shortcomings is a sign of goodness, and a Muslim must acquire abundant good deeds to overcome the impulses of evil.