What is the act of worship that removes the Major sins ?
The repentance (At-Tawbah).
Allah says in his book :
And verily, I am indeed Forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them, (till his death). (Taha: 82)
And doing lot of good deeds and acts of worship. Allah says in his book :
O Ibadee (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful. (Az-Zumar : 53)
The condition of Repentance from a sin are :
1. The repentance should be for the sake of Allah
2. Refrain from that sin
3. To make strong determination to not return to that sin
4. If the sin is between him an another person like taking their rights then he need to return their rights to them.
And the repentance should be before a person in the mouth of death (Gargarah) or before the sun raises from west as informed by Prophet Sallallahu alaihi wa sallam.
Answered by : Shaykh Abu Ishaaq Al Qaisee