Question Summary:
How to calculate forgotten/unrecorded interest amounts in a bank account & harms of interest in this world

Question Detail:

Assalamu Alaikum,
 
Few Months back, i came to know that my father never in his life disposed interest money of his saving accounts so as to prevent his halal earned money from getting mixed with haram interest.
He aged 72 now. He had been working since 1961. First time he opened saving bank account in 1965.
Between 1965 and 2001 he had opened 5 saving bank accounts. He doesn't has details of these accounts now.
Since 2001 he has only one saving account which he uses till now.
He has not kept track of interest money from his accounts all these years. 
1. I know allah cannot be happy with a person involved with interest. such a person has very harsh punishment in aakhirah. I want my father should be clear of liability towards interest money before he meets allah.
Please help me how do i calculate his interest money of all these years. Note: we don't have accounts of transactions from 1965 till 2000s.
2. I know bank interest is haram but i want to know why? Can you please give me a link to a detailed article from any trusted scholar.
3. How adverse is to be indulged even a single penny of interest. What are harms of in this world of dealing with interest.
4. One thing i have observed in me, all of my brothers and sisters that we (specially me) have been disobedient to our parents all these years. 
We knew that it is wrong to speak ill, loud, harsh, false, bad words etc to parents, We were not able to keep ourself within limit of "Wa la takallahuma uff". 
I think this haram interest money was one of the reason for we children not being completely obedient to our parents. Does interest money has such effect?
 
May allah bless you all

Answer :

• You are aware that interest is Haram. You are also aware that interest incurs the wrath of Allah. It is this belief and conviction that has led you to enquire how to cleanse your father’s wealth from the interest. In point three of your query you enquire about the harms of interest.

Consider the following verses:

Those who consume interest cannot stand on the day of resurrection except as one stands who is being beaten by satan into insanity. 
 

 And Allah has permitted trade and forbidden interest 
 

O you who have Iman! Fear Allah and give up what interest is left (what is still due to you) if you are true believers. If you do not then be informed of war against you from Allah and his messenger 
 
 
Also consider the following ahadith:

Indulging in a single penny of interest intentionally is worse than committing adultery 36 times. 
 

The Messenger of Allah (ﷺ) cursed the one who accepts interest and the one who pays it. Narration of muslim.
 

It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:
"There are seventy degrees of usury, the least of which is equivalent to a man having intercourse with his mother."
 
If details of the bank account are not available, we advise you consult with an accountant who will help you to estimate the interest accumulated according to the general bank policy and general percentage of interest earned from 1961 till 2000. It is advisable to make an overestimate of the amount and dispose of it in sadqa by giving it to the poor and needy.
The interest earned in the present banking and savings account should also be calculated of and deposited.
 

http://darulfiqh.com/the-severity-of-interest-in-the-quran-and-ahaadith/
 

 
 
And Allah Ta’āla Knows Best
Aboobakr Siddeeq bin Mufti Amjad Mohammad
Student Darul Iftaa
Azaadville, South Africa 
Checked and Approved by,
Mufti Ebrahim Desai.