Restoring One’s Iman
A small excerpt from the book:
It is necessary for every servant who has ʾĪmān and sincerity with himself to inspect his ʾĪmān; does his ʾĪmān grow, or does it wither?
The servant is in dire need to restore his ʾĪmān, take steps in perfecting it, being on one's guard from things that decrease and weaken it, and make strides to strengthen it.
ʾĪmān is subjected to afflictions by numerous matters that decrease it; among them are trials of this life, Shayṭān’s whispers, bad and corrupt companions, an evil soul that always calls to evil and so forth. So, one needs to take quick measures in strengthening and restoring it.
Al-Ḥākim reported in his book al-Mustadrak and at-Tabarānī reported in his book al-Mʿujam al-Kabīr on the authority of ʿAbdullāh bin ʿUmar bin al-Aas (radiyallahu 'anhu) said that the Messenger of Allāh (sallallahu' alayhi wa sallam) said,
إِنَّ الْإِيـمَـانَ لَـيَـخْـلَـقُ فِـي جَـوْفِ أَحَـدِكُـمْ كَـمَـا يَـخْـلَـقُ الـثَّـوْبُ ، فَـاسْـأَلُـوا اللهَ أَنْ يُـجَـدِّدَ الْإِيـمَـانَ مْيقُـلُـ
Indeed, the ʾĪmān in your heart wears out similarly to how a Thawb wears. So beg Allāh to restore the Iman in your hearts. ”
The Prophet (sallallahu 'alayhi wa sallam) described ʾĪmān as to wear out the same way as a Thawb wears out, meaning it is subjected to deterioration, weakness, decrease due to disobedience, sins, and conflicting diversions that keep him away from ʾĪmān, and hard trials that dwindle the strength and vigor of his ʾĪmān; and weakens its beauty, excellence, and splendor.
 In the book al-Mustadrak (4/1) and the book al-Mʿujam al-Kabīr (36/13); and Shaykh al-Albānī authenticated it in his book as-Silsilah as-Ṣaḥīḥah (1585).