The Messenger (sallal-laahu-alayhi-wasallam) said:

إِنَّ الْإِيمَانَ لَيَخْلَقُ فِي جَوْفِ أَحَدِكُمْ كَمَا يَخْلَقُ الثَّوْبُ فَاسْأَلُوا اللَّهَ أَنْ يُجَدِّدَ الْإِيمَانَ فِي قُلُوبِكُمْ

‘’Indeed, Eemaan wears out in the heart of one of you just as the thawb (i.e. clothes) wears out, so ask Allaah to renew Eemaan in your hearts.’’ [Source: Silsilah As-Saheehah –[Hadeeth No 1585  (Vol 4 page 113)]

The Creed of Ahlus Sunnah Wal Jamaa’ah –[Imaan Increases and Decreases]

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http://www.salafipublications.com/sps/sp.cfm?subsecID=GSC05&articleID=GSC050001&articlePages=1