The Prophet (ﷺ) said:”Eman means that you believe in Allah, in His Angels, in His Books, in His Messengers, in the Day of Judgment and in the Divine Decree (Qadr) in its good and evil.”[Sahih Muslim 8a]

Abu Ja’far reported: ‘Umayr ibn Habib, may Allah be pleased with him, said, “Faith increases and decreases.” It was said, “How does it increase and decrease?” ‘Umayr said, “If we remember our Lord and fear him, it will increase. If we are heedless and we forget and we waste our time,  it will decrease.”

Source: Shu’ab al-Imān 55

عَنْ أَبِي جَعْفَرٍ الْخَطْمِيِّ عُمَيْرِ بْنِ حَبِيبِ رضي الله عنه أَنَّهُ قَالَ الإِيمَانُ يَزِيدُ وَيَنْقُصُ فَقِيلَ لَهُ وَمَا زِيَادَتُهُ وَمَا نُقْصَانُهُ قَالَ إِذَا ذَكَرْنَا رَبَّنَا وَخَشَيْنَاهُ فَذَلِكَ زِيَادَتُهُ وَإِذَا غَفَلْنَا وَنَسِينَا وَضَيَّعْنَا فَذَلِكَ نُقْصَانُهُ

55 شعب الإيمان للبيهقي باب القول في زيادة الإيمان ونقصانه

• 1. Belief in Allah:This includes believing in Allah’s existence, His oneness (Tawhid), His attributes, His Lordship, and His right to be worshipped alone. 
• 2. Belief in Angels:This involves believing in the existence of angels as Allah’s messengers, their obedience to Him, and their specific roles. 
• 3. Belief in Allah’s Books:Muslims believe in the divine books revealed by Allah to various prophets, including the Quran, which is considered the final and complete revelation. 
• 4. Belief in Allah’s Messengers:This pillar encompasses belief in all prophets sent by Allah, including Adam, Noah, Abraham, Moses, Jesus, and Muhammad (peace be upon them all). 
• 5. Belief in the Last Day:This is the belief in the Day of Judgment, when all of humanity will be resurrected and judged by Allah based on their deeds. 
• 6. Belief in Divine Decree (Qadar):This pillar entails believing in Allah’s predestination of all things, both good and bad, and that everything happens according to His will and wisdom.