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Whoever sees sincerity in his sincerity, his sincerity is itself in need of sincerity. The destruction of every sincere person lies in his sincerity, (he is destroyed) to the extent that he sees sincerity in himself. When he abandons seeing sincerity in himself he will be sincere and purified.
- A Salaf (radiAllahu 'anhu)

Ikhlas -Sincerity- is to forget the vision of creation
by constantly looking at the Creator.
- Abu 'Uthman (radiAllahu 'anhu)

Only someone who is sincere recognizes what showing off is.
- Imam Ash-Shafi'i (rahimullah)

There are two kinds of Riya -Showing off- Ostentation i.e. pure ostentation and adulterated ostentation. In pure ostentation "Riya" a man does a good deed only for worldly benefit. In Adulterated ostentation, a man does a good deed with the intention of reaping the benefits of the world as well as of the Hereafter.
- Imam Abu Hamid Al Ghazali

Innumerable changes of moods are yours,
and they are uncontrolled by you.
If you knew their origin,
you would be able to dominate them.
If you cannot localize your own changes,
how can you localize that which formed you?
- Ash-Shaykh Jalal'uddin al-Rumi qs [Fihi Ma Fihi]

If the first inward thought is not warded off, it will generate a desire, then the desire will generate a wish, and the wish will generate an intention, and the intention will generate the action, and the action will result in ruin and divine wrath. So evil must be cut off at its root, which is when it is simply a thought that crosses the mind, from which all the other things follow on.
- Imam Abu Hamid al-Ghazali [Ihyaa Uloom al-Deen, 6/17]