One Who Repents from A Sin is Like One Who Has No Sin

Question :

For months at a time, I wouldn't pray; Ramadhan would come and go, yet I wouldn't fast; what I would do frequently was smoking cigarettes. All praise is due to Allah, I have repented from all of these sins - I pray all of the prayers on time now, I fast Ramadhan, and I have quit smoking. My question is this: Must I make up for all of the prayers I had missed in the past?

Answer:

First, because Allah Almighty has granted you repentance and because you now perform your obligatory Islamic duties, I congratulate you, and I ask Allah Almighty to make you firm on this path, to increase for you from His blessings, to make us all die upon faith, and to raise us all in the company of the noblest human being, Muhammad.

Second, your repentance nullifies previous sins, including the prayers and fasts you neglected to perform, for Allah Almighty says:
"Say: O My servants who have transgressed against themselves! Despair not of the Mercy of Allah, verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful."

"And those who, when they have committed Fahishah (illegal sexual intercourse etc.) or wronged themselves with evil, remember Allah and ask forgiveness for their sins; and none can forgive sins but Allah - And do not persist in what (wrong) they have done, while they know. For such, the reward is Forgiveness from their Lord, and Gardens with rivers flowing underneath (Paradise), wherein they shall abide forever. How excellent is this reward for the doers (of righteous deeds)."

Based on the rulings mentioned in this verse, you do not have to make up for prayers and fasts you neglected to perform in the past; however, continue to repent, to ask Allah Almighty for forgiveness, and to perform good deeds; indeed Allâh Almighty forgives those who repent.

Source:
Ibn 'Uthaimeen
Fatawa Islamiyah, Vol. 7 Pages 259-260