Ibn Majah

Commentary:
(1)
Although both of these narrations are weak in their chains of transmission and are not established as marfu‘ (directly attributed to the Prophet sallallahu alayhi wa sallam), this matter has been transmitted in a mutawatir (continuous and widespread) manner from the Salaf and is well-known; therefore, increase and decrease in faith (iman) is accepted among Ahl al-Sunnah.
Imam al-Bukhari rahimahullah, in his book Sahih al-Bukhari, in the very first chapter of Kitab al-Iman, has mentioned several verses of the Noble Qur’an () as proof of this, and after that, in several chapters, he has mentioned such ahadith from which it is established that righteous deeds are components (ajza’) of faith.
It should be clear that if something has components, then if one or several of those components are missing, that thing becomes deficient.
For further details on this issue, studying the relevant chapters of Fath al-Bari will be beneficial.