Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

In the Name of Allah, the All-Merciful, the Mercy-Giving

All praise is due to Allah. Peace and blessings be upon His Messenger Muhammad.

In answering this question, I cannot do any better than citing from one of my earlier answer on a similar question:

“Salah is the most important pillar of Islam. It is in fact the first religious duty prescribed on every single prophet from Adam to Muhammad, second only to testifying the Oneness of Allah. Allah says, “Salah, indeed, is a duty enjoined upon the faithful at the appointed times.” (An-Nisa’: 103). The Prophet (peace and blessings be upon him) further narrates in a hadith Qudsi: Allah said: “The covenant between Us and them is Salah; so whoever establishes it establishes religion; whoever undermines it undermines religion.”

It is therefore important that we never become slack in performing salah; if we miss any salah for whatever reason, we must pray it immediately without further delay. The Prophet (peace and blessings be upon him) said, “Whoever oversleeps and misses his prayer or forgets to do it, let him pray as soon as he remembers it.”

In light of these and other evidence, the vast majority of scholars and imams are of the opinion that one must make up for all of the prayers one has missed in life, no matter how many they are. So according to them, you should make up for all of these prayers. One of the best ways to do this—as has been suggested by one scholar—is to pray with each fard that you perform another fard in lieu of what you missed in the past. Thus, for instance, before or after praying Zuhr, pray another four rak`ahs of Zuhr as qadha’, and pray another four rak`ahs of `Asr every time you pray `Asr; you should continue to do this until such time that you can be pretty sure that you have made up for all of the missed prayers.

However, the above view has been rejected by scholars such as Imam Ibn Taymiyah, Shawkani, and Ibn Hazm. They are of the view that a person who has deliberately missed his prayers can never make up for them. Therefore, the only option left for him is to repent, ask forgiveness of Allah, and do lots of good works; by doing so he can hope to receive Allah’s mercy.

Ibn Taymiyyah, while advancing this point of view, further states: “To insist that a person who has strayed away from Islam for a number of years and then returns to the fold of Islam must make qadha’ of all his missed prayers serves only as a deterrent against his repentance, and thus it amounts to limiting the infinite mercy of Allah.” He, therefore, dismisses this view and rules that it is sufficient for him to repent, make lots of istighfar (asking forgiveness) and good works.

Having said this, I must add that you must never be slack again in your prayers.

May Allah shower us all with His mercy and help us to remain steadfast in His religion. Ameen.”

Almighty Allah knows best.