Prophet Muhammad S.A.W said, "Temptations are presented to the heart, one by one. Any heart that accepts them will be left with a black stain, but any heart that rejects them will be left with a mark of purity, so that hearts are of two types: a dark heart that has turned away and become like an overturned vessel, and a pure heart that will never be harmed by temptation for as long as the earth and the heavens exist. The dark heart only recognises good and denounces evil when this suits its desires and whims." (Sahih Muslim, Kitab al-Iman, 2/170 (with different wording).
"He it is Who sent down calmness and tranquillity into the hearts of the believers, that they may grow more in Faith along with their (present) Faith. And to Allah belong the hosts of the heavens and the earth, and Allah is Ever Al-Knower, All-Wise." The Holy Quran: 48:4
http://www.sunnah.org/tasawwuf/hearts.htm
"ketahuilah di dalam jasad ada seketul daging, jika baik ia maka baikalah seluruh jasadnya, jika buruk ia, maka buruklah jasad seluruhnya, ketahuilah ia adalah hati(qalb)"
"He it is Who sent down calmness and tranquillity into the hearts of the believers, that they may grow more in Faith along with their (present) Faith. And to Allah belong the hosts of the heavens and the earth, and Allah is Ever Al-Knower, All-Wise." The Holy Quran: 48:4
http://www.sunnah.org/tasawwuf/hearts.htm
"ketahuilah di dalam jasad ada seketul daging, jika baik ia maka baikalah seluruh jasadnya, jika buruk ia, maka buruklah jasad seluruhnya, ketahuilah ia adalah hati(qalb)"