Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
It is established in the Qur’an and Sunnah that people will receive their records of deeds on the Day of Judgment. Some will receive their record in their right hand, while others will receive it in their left hand or from behind their backs.
Allah says:
“As for he who is given his record in his right hand, he will say, ‘Here, read my record! Indeed, I was certain that I would meet my account.’ So he will be in a pleasant life, in an elevated Paradise.” [Al-Haqqah 69:19–22]
And He says concerning the people of the left:
“But as for he who is given his record from behind his back, he will cry out for destruction and enter the Blaze. Indeed, he was joyful among his people, and he thought that he would never return.” [Al-Inshiqaq 84:10–14]
The context indicates that receiving the record in the left hand or from behind the back is a characteristic of the disbelievers, as Allah says:
“And those who disbelieve in Our signs—they are the people of the left.” [Al-Balad 90:19]
As for the sinful believer who dies upon monotheism without having repented from all his sins, the scholars differed concerning how he will receive his record. Some said that he will receive it in his right hand, while others said that he will receive it in his left hand.
A number of scholars, including Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him), preferred the view that he will receive his record in his right hand, because the apparent meaning of the texts indicates that receiving it in the left hand is specific to the disbeliever.
However, receiving the record in the right hand does not necessarily mean that the Muslim will not be punished for any of his sins. A Muslim may be a monotheist, receive his record in his right hand, and then Allah may either forgive him or punish him for whatever sins He wills. Ultimately, his destination will be Paradise.
It has been authentically reported that Allah will draw the believer near on the Day of Judgment, conceal him, and remind him of his sins. Then He will say to him: “I concealed them for you in this world, and today I forgive them for you.” Then he will be given his record of good deeds in his right hand.
As for the disbeliever, he will be exposed before all creation, and his record will be given to him in his left hand or from behind his back.
In summary: Receiving the record in the right hand is generally one of the signs of the people of faith and salvation, but it does not necessarily mean that everyone among the people of the right will be punished for none of their sins. A sinful Muslim will not remain eternally in Hell if he dies upon monotheism.
A Muslim should strive in acts of obedience, stay away from sins, and ask Allah to make him among the people of the right.
And Allah knows best.