Ibne Mehboob has mentioned in his 'Sahih' that he and his companions had written a letter to Imam Rida (A.S) enquiring about the number of the Greater Sins. Also the method of determining their "greatness". The respected Imam (A.S) replied thus:
"The Greater Sins are those, the performers of which have been promised (Hell) fire. If any believer abstains from these or repents for the same Allah will forgive these sins. There are seven such sins, which have been promised (Divine) punishment.
1. To kill one whose murder is prohibited.
2. To disobey parents.
3. To accept usury.
4. To return to ignorance after migration (Hijrat).
5. To accuse a chaste woman of adultery.
6. To consume wrongfully the property of orphans.
7. To flee from the battlefield.
Abu Samit has narrated from Imam Sadiq (A.S) that he said:
The Greater Sins are:
1. To associate others (attribute partners) to Allah.
2. Killing anyone whose killing is prohibited by Shariat.
3. Wrongfully acquiring the property of orphans.
4. To disobey parents.
5. To accuse falsely a chaste woman of adultery.
6. To flee (cowardly) from the battlefield.
7. To deny any of the obligations of religion promulgated by Allah.
Abdur Rehman Ibne Katheer has narrated from the same Masoom (A.S) that he said:
"Not acknowledging the rights of us (Ahl-ul-Bayt) is a Greater sin."
Abi Khadija has quoted a tradition that:
"To attribute falsehood to the Holy Prophet (peace be upon him and his household) and the Holy Imams (A.S) is counted among the Greater Sins."
Sadooq (r.a) has quoted that: "To deprive anyone of his rights (What he rightfully deserves) by not mentioning him in one's will is a Greater sin."
The book Kanzul Ummal has mentioned in a tradition:
"Shedding blood in the precincts of the Holy Kaba is a Greater sin."
This is because Allah has ordained it to be a protected sanctuary.
In the Oyoon-e-Akhbaare Rida there is a tradition quoted by Fazl ibne Shazaan. He quotes from reliable authorities that Imam Rida (A.S) wrote to Mamun a letter prohibiting the following sinful acts, as they were Greater Sins:
1) To kill those whose murder is prohibited by Allah.
4) Drinking of Wine.
5) To disobey the parents.
6) Fleeing from the battlefield (in Jihad).
7) Usurping property of the orphans.
8) Eating dead animals (animals not slaughtered in the Islamic way) blood and pigs without any compulsion.
9) Accepting usury when it is obvious.
10) Consuming prohibited (Haraam) wealth.
12) Cheating in business by under-weighing.
13) To accuse chaste women of adultery.
15) To despair of the Mercy of Allah.
16) Not fearing the Divine retribution.
17) Not acknowledging the bounties of Allah.
18) Cooperating with oppressors.
19) To associate with the oppressors.
20) Taking a false oath.
21) To obstruct the rights (of someone) without (any valid reason or compulsion).
23) Pride (arrogance).
24) Squandering wealth.
25) Expenditure in ways not permitted by Allah.
26) To betray trust.
27) To consider Hajj of the Holy Kabaa unimportant.
28) To fight the friends (devotees) of Allah.
29) To enjoy vain preoccupations (like music).
30) To persist in sins.
* The book named: GREATER SINS. By Al-Haaj Sayyed Abdul Hussein Dastghaib Shirazi (r.a)