A person once asked The Prophet Muhammad PBUH.
What is faith?
The Prophet replied.
When a good deed becomes a source of pleasure for you and an evil deed becomes a source of disgust then you are a believer.
He was then asked.
What is a sin?
The Prophet said.
When something pricks your conscience give it up.
Al-Tirmidhi Hadith 8.
Briefly Commentary taken from Tafsir Ibn e Kathir.
Allah said.
ﺍﻟَّﺬِﻳﻦَ ﺀَﺍﻣَﻨُﻮﺍْ ﻭَﺗَﻄْﻤَﺌِﻦُّ ﻗُﻠُﻮﺑُﻬُﻢْ ﺑِﺬِﻛْﺮِ ﺍﻟﻠَّﻪِ.
Those who believed and whose hearts find rest in the remembrance of Allah.
For their hearts find comfort on the side of Allah become tranquil when He is remembered and pleased to have Him as their Protector and Supporter.
So Allah said.
ﺃَﻻَ ﺑِﺬِﻛْﺮِ ﺍﻟﻠَّﻪِ ﺗَﻄْﻤَﺌِﻦُّ ﺍﻟْﻘُﻠُﻮﺏُ.
Verily in the remembrance of Allah do hearts find rest.
And surely He is worthy of it.
Everything that hearts find of worries and sadness is because of what they have been denied of beholding Allah.
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