Hasan Basri said :

Al-Hasan al-Basri (may Allah have mercy on him) said:

Whoever competes with you in religious commitment, compete with him, and whoever competes with you in worldly matters, give it up to him.

Imam Malik said :

Imam Malik, said: “The Sunnah is like the ark of Noah. Whoever embarks upon it achieves salvation and whoever rejects it, is drowned”.

Imam Ibn ul Qayyim al Jawziyyah
There are ten useless matters:

Knowledge that is not acted on

The deed that has neither sincerity nor is based on following the righteous examples of others

Money that is hoarded, as the owner neither enjoys it during this life nor obtains any reward for it in the Hereafter

The heart that is empty of love and longing for Allah, and of seeking closeness to Him

A body that does not obey and serve Allah

Loving Allah without following His orders or seeking His pleasure

Time that is not spent in expiating sins or seizing opportunities to do good

A mind that thinks about useless matters

Serving those who do not bring you close to Allah, nor benefit you in your life

Hoping and fearing whoever is under the authority of Allah and in His hand; while he cannot bring any benefit or harm to himself, nor death, nor life; nor can he resurrect himself.

However the greater of these matters are wasting the heart and wasting time. Wasting the heart is done by preferring this worldly life over the Hereafter, and wasting time is done by having incessant hope. Destruction occurs by following one's desires and having incessant hope, while all goodness is found in following the right path and preparing oneself to meet Allah.

How strange it is that when a servant of Allah has a [worldly] problem, he seeks help of Allah, but he never asks Allah to cure his heart before it dies of ignorance, neglect, fulfilling one's desires and being involved in innovations. Indeed, when the heart dies, he will never feel the significance or impact of his sins.

"The wealth of Fuqara".

Saying of Rumi

Whenever you are inclined to undertake some particular work, you recognize your own freedom to do so. But when you are not inclined to undertake some particular work, you are fatalistic, saying : "God requires me to do this."

Prophets are fatalists regarding the works of this world; unbelievers are fatalists regarding the works of the next world. The prophets proclaim that we can choose freely what we shall do in the next world; unbelievers choose freely what they will do in this world.

- Rumi : Mathnawi 1

Ibn Sina said :

If we accept the propositions concerning predestination as valid, we need to add that God does not intend evil. The apparent evil which occurs in this world do not conform to God's purpose for the world. The purpose for which the world exists, is the triumph of good ; Evil is the frustration of that purpose.

- Ibn Sina - Kitab-al-Najat

Ali RA said

Ali RA said , "Among deeds, there are four that are most difficult to adorn oneself with :

1. Forgiveness when angry
2. Generosity in hard times
3. Chastity when alone and
4. Speaking the truth to the one that fears it.

Razi said :

Learned people who fail to restrain their passions, are like blind people carrying torches, they guide others, but not themselves

Umar RA said,

It is attributed to Umar ibn al khattab radhiAllah anhu that he said, 'Take account of your deeds before the account is taken from you'.

Another saying of Umar radhiAllah anhu that i like is, he said that best reply/retaliation to an injustice done to you and a disobedience to Allah subhanahu wa ta'ala's command regarding you is, you sticking with the laws of Allah subhanahu wa ta'ala and obeying Him to best of your capacity.