Hadith: Do you not testify that there is no god but Allah and that Muhammad is the Messenger of Allah?

عَنْ أَنَسٍ قَالَ: جَاءَ رَجُلٌ إِلَى النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: يَا رَسُولَ اللَّهِ مَا تَرَكْتُ حَاجَةً وَلا دَاجَةً إِلا قَدْ أَتَيْتُ. قَالَ: "أَلَيْسَ تَشْهَدُ أَنْ لَا إِلَهَ إِلا اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ" ثَلاث مَرَّاتٍ قَالَ: نَعَمْ قَالَ: "فَإِنَّ ذَلِك يَأْتِي على ذَلِك".
[صحيح] - [رواه أبو يعلى والطبراني والضياء المقدسي]

Anas (may Allah be pleased with him) reported: A man came to the Prophet (may Allah’s peace and blessings be upon him) and said: "O Messenger of Allah, I left no minor or major sin without committing it." He said: "Do you not testify that there is no god but Allah and that Muhammad is the Messenger of Allah?" He repeated the question three times. He replied: 'Yes.' The Prophet (may Allah’s peace and blessings be upon him) said: "This prevails that."
Sahih/Authentic. - [Abu Ya‘laa]

Explanation

A man came to the Prophet (may Allah’s peace and blessings be upon him) and said: O Messenger of Allah, I committed all sins and evil acts, minor and major. Is repentance possible for me? The Prophet (may Allah’s peace and blessings be upon him) said: Do you not testify that there is no god but Allah and that Muhammad is the Messenger of Allah? He repeated the question three times. The man replied: "Yes, I testify." Thereupon, he said: The two testimonies prevail sins and overcome them, and Islam and the Tawhīd (monotheism) expiate the sins a person committed as a disbeliever. With regard to the Hadīth, "Will we be punished for what we did in the Jāhiliyyah (pre-Islamic period of ignorance)?" The Prophet (may Allah’s peace and blessings be upon him) said: "Whoever does good in Islam will not be punished for what he did in the Jāhiliyyah and whoever does evil in Islam will be punished for the former and latter", the evil here means apostasy, which indicates that a person's sins can be expiated by virtue of embracing Islam only if he dies as a Muslim.