Can you provide the reference for this as well as the Arabic text?
Hassan Al Basri (rahimahullah) said:
A believer is the one who, after performing a good deed, is fearful and a transgressor is the one who, after committing a sin, is at peace
Hafiz Ibn Kathir (rahimahullah) has cited this statement of Hasan Al Basri (rahimahullah) without citing a source. I have not come across this narration in any Primary source.
(Tafsir Ibn Kathir, Surah Al A’araf, Verse: 99)
قال الحسن رحمه الله تعالى: المؤمن يعمل بالطاعات، وهو مشفق وجل خائف. والفاجر يعمل بالمعاصي وهو آمن
Hasan Al Basri (rahimahullah) said:
“A Believer carries out good deeds and is fearful [of the fact that his deeds may not be accepted] and a sinner carries out sins and is at ease/peace [does not fear accountability].”
Imam Bukhari (rahimahullah) has recorded a statement of Sayyiduna ‘Abdullah ibn Mas’ud (radiyallahu ‘anhu) wherein he expresses that a believer does not trivialise his sins and a sinner regards his sins as extremely trivial.
Sayyiduna ‘Abdullah ibn Mas’ud (radiyallahu ‘anhu) said: “A believer views his sins as if he is sitting under a mountain which he is afraid may fall on him whereas the sinner views his sins like a fly passing by his nose.” The narrator then passed his hand over his nose to illustrate this.
(Sahih Bukhari, Hadith: 6308)
And Allah Ta’ala Knows best.
Answered by: Moulana Suhail Motala
Approved by: Moulana Muhammad Abasoomar
This answer was collected from HadithAnswers.com. The answers were either answered or checked by Moulana Haroon Abasoomar (rahimahullah) who was a Shaykhul Hadith in South Africa, or by his son, Moulana Muhammad Abasoomer (hafizahullah), who is a Hadith specialist.