In the Holy Qur'an
Most surely there is a reminder in this for him who has a heart or listens while he is present (50:37)
And truly he (David) has a intimacy with Us and a good resting place (38:25)
In the Traditions
The Prophet said, "Allah, most High, has receptacles on the earth, and verily they are the hearts. The most beloved of hearts to Allah are the softest ones, the purest ones, and the firmest ones: those that are the softest to their brothers, those that are the most pure from sins, and those that are the firmest in the path of Allah."
Imam Ali said, "Tears only dry up (ie. fail to flow) as a result of hardness of the heart, and the hearts only harden as a result of frequent sinning."
When a person complained to the Prophet about the hardness of his own hear, the Prophet replied, "If you want your heart to soften, feed the needy and stoke the head of ophans."
Imam al-Jawad said, "Seeking Allah with the hearts is more effective than tiring the limbs with actions"