Surah 37. As-Saffat, Ayah 84

A sound heart: qalb salim: a heart that is pure, and unaffected by the diseases that afflict others. As the heart in Arabic is taken to be not only the seat of feelings and affections, but also of intelligence and resulting action, it implies the whole character. Cf. Abraham's title of Hanif (the True): ii. 135 and n. 134. Cf. also xxvi. 89: "only he will prosper who brings to Allah a sound heart."