Superiority of the words (There is no God but AIHih)
101) Narrated by'Abdullah bin Amr bin AI-' Aas that Allah's Messenger said: On the Day of Resurrection, Allah will bring a person of my Ummah from among the people; and spread before him ninety-nine registers (filled up with his deeds). Each of them will be as long as sight extends. Then Allah will ask him: Do you deny anything of it? Have My preserving writers treated you unjustly? The person will reply: No, my Lord. Allah will further ask him: Do you have any excuse (for not having good deeds)? He will say: No, my Lord. Then Allah will observe: Behold! We have preserved a virtue of yours with us. Today there will be no wrong with you. Then Allah will take out a slip containing the words: "There is no God but Allah and that Muhammad is His slave and Messenger"; and Allah will address the person: Bring up your weights (of this slip). The person will remark: 0 Lord! This slip (is so light in weight) that it may not be matched with these large registers. Allah will observe: You will be done no injustice. The Prophet added: All the registers will be piled up in one scale and the slip will be kept in the other scale of the balance. (With the Grace of Allah), the scale with the registers will be raised up (due to light weight) while the scale having the slip will be the heavier as there is nothing that weighs more than the Name of Allah.
(This Haditli is sound and reported by Tirmidhi, Ibn Majah and Ahmad in his Musnad).
This above Hadith briefly describes that Allah, if He likes, can forgive even the most sinner person believing in Oneness of Allah and the Prophethood of Muhammad . But it doesn't mean that everyone should take it as a cover and involve himself intentionally in evils relying on the Mercy of Allah. A good Muslim who has in his heart fear of Allah, should perform righteous deeds and avoid sins as per commands laid down in the Qur'an and Hadith.