As one finishes his ablution and goes to prayer, one would believe that his outer appearance is clean and pure, and that is what [the aspect that] other people see. But one should feel ashamed to have a discourse with Allah [Prayer is essentially and in its finest practice, a deep experience of having a kind of communication with God] without having cleaned and purified his Heart [one’s Essence], and that is what Allah SWT sees [probably pointing to the Hadith that “Allah does not look at your (physical) faces but at your hearts.”].
So, one should aspire to have a clean and pure heart by repentance [assuming that one might have behaved or said something that might have ‘stained the heart’, since the last prayer]. Although, not having bad/negative traits and having fine manners is better and should be one’s priority [in an attempt to reach a state of sustained purity of the heart].
Whoever is [going to prayer] content with having a clean outer appearance, is like someone inviting a king to his house, and he is busy with beautifying the outer part / the entrance to the house, while the house is full of garbage.
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