A person who, before his repentance, did not know how many prayers and months of fasting he had missed: what do you advise him?

Question:
The first question says: There is a person who, in the early part of his life, prayed and fasted little, but now he is moderate, and he regrets what has passed, knowing that he does not know the number of years he did not fast. What is the ruling? And what do you advise him? May Allah reward you with good.

Answer:
In the name of Allah, the Most Gracious, the Most Merciful. Praise be to Allah, Lord of the worlds, and may Allah send prayers and blessings upon our Prophet Muhammad, and upon his family and all his Companions. As for what follows, prayer and fasting are two of the pillars of Islam. Prayer is the second pillar after the two testimonies, and fasting is the fourth pillar of Islam. It is obligatory for the Muslim to perform the five pillars of Islam, to fulfill the other obligations, to avoid the forbidden, sins, and evil deeds, and to increase in voluntary acts of obedience after the obligatory ones. This is what is required of the Muslim, and it is his provision that he will find with Allah, Glorified and Exalted is He, . If a person has been negligent in his prayer or his fasting, it is obligatory for him to repent, because intentionally abandoning prayer is apostasy from Islam, and abandoning fasting is likewise a deficiency in the Muslim’s religion, a deficiency in his Islam. So this questioner must repent to Allah, Mighty and Majestic, with a sincere repentance, and maintain the matters of his religion, especially these pillars. He must make up the fasting he left off, if he was praying during the period in which he left off fasting; in that case, he makes up the fasting and feeds one poor person for every day. If he does not know the number of days he did not fast, he estimates it, strives to estimate it, and takes precautions. As for if he was not praying during the period in which he left off fasting, then he was not a Muslim during that period. It is obligatory for him to repent to Allah and enter Islam anew, and to commit to fasting, prayer, and the pillars of Islam in the future of his life. Repentance annuls what came before it, because even if he fasted during that period while not praying, his fasting is not valid, because fasting and all other acts of obedience are not valid except with adherence to the two testimonies and prayer, so that a person is a Muslim whose worship is valid. As for if he has a defect in his creed due to committing major shirk (associating partners with Allah), or if he is one who abandons prayer, he is not considered a Muslim except from the time he repents to Allah with a sincere repentance and declares his Islam anew, for repentance annuls what came before it.