Is there proof in the Sunnah or the Qur’an indicating the need to make up missed prayers for one who has abandoned prayer for years, and if there is no proof, what should he do to seek forgiveness and avoid being held accountable for them on the Day of Resurrection?

Repentance must be immediate, for abandoning prayer is one of the gravest sins. Ibn al-Qayyim reported that Muslims do not differ on the fact that intentionally abandoning prayer is among the greatest sins, and that it is more sinful than murder, adultery, theft, and consuming alcohol.

All missed prayers must be made up, and this is the view of the majority of scholars, who cite the hadith, "The debt owed to Allah is more deserving of being fulfilled," and the Prophet’s (peace be upon him) command to make up prayers for those excused by sleep or forgetfulness. Therefore, making up prayers is even more incumbent upon the intentional transgressor.

The making up of prayers should be done in a way that does not harm one's body or livelihood, for Allah does not burden a soul beyond its capacity.