The major sins are those acts which have been forbidden by Allah in the Quran and by His Messenger (may Allah bless him and grant him peace) in the Sunnah (practise of the Prophet), and which have been made clear by the actions of of the first righteous generation of Muslims, the Companions of the Prophet (may Allah bless him and grant him peace)
There is some difference of opinion among scholars in this regard. Some say these major sins are seven, and in support of their position they quote the tradition: ‘Avoid the seven noxious things’- and after having said this, the propeht (may Allah bless him and grant him peace) mentioned them: ‘associating anything with Allah; magic; killing one whom Allah has declared inviolate without a just case, consuming the property of an orphan, devouring usury, turning back when the army advances, and slandering chaste women who are believers but indiscreet.’ (Bukhari and Muslim)
‘Abdullah ibn ‘Abbas said: ‘Seventy is closer to their number than seven,’ and in this book Imam Dhahabi goes through the 70 Major Sins Supported by the Qur’an and the Sunnah of the Prophet Muhammad (may Allah bless him and grant him peace).
The Author Muhammad bin Ahmad bin `Uthman bin Qaymaz at Turkamani, Shams al-Din al-Dimashqi al-Dhahabi al-Shafi`i (673-748 AH), the imam, Shaykh al-Islam, head of hadith masters, critic and expert examiner of the hadith, encyclopedic historian and biographer, and foremost authority in the canonical readings of the Qur’an. Born in Damascus where his family lived from the time of his grandfather `Uthman, he sometimes identified himself as Ibn al-Dhahabi – son of the goldsmith – in reference to his father’s profession.