'Khalid ibn al-Walid said, "O Messenger of Allah, shall I kill him?"
He said, "Perhaps he is one of those who pray." Khalid said, "How many people say with their tongues what is not in their hearts?"
The Prophet said, "I have not been ordered to look into the hearts of people, nor to rip open their bellies." (Abridged from al-Bukhari and Muslim.)'
we might needs to go for the arabic for the words 'heart', since there are so many words, standing on the consciousness of the heart:
إِقْدام ؛ بَطْن ؛ بُهْرَة ؛ باطِن ؛ باطِنَة ؛ بال ؛ جَأش ؛ جَسَارَة ؛ جَنَان ؛ جَوْف ؛ جَوْهَر ؛ جَيْب ؛ خاطِر ؛ دَخِيلَة ؛ دَم ؛ داخِل ؛ داخِلَة ؛ رُوح ؛ رُوع ؛ سَرِيرَة ؛ صَدْر ؛ صَمِيم ؛ صُلْب ؛ ضَمِير ؛ طَوِيّة ؛ عَقْل ؛ فُوَاد ؛ فُؤَاد ؛ فِكْر ؛ قَلْب ؛ قَلْبِيّ ؛ كَبِد ؛ لُبّ ؛ مَرْكَز ؛ مُنْتَصَف ؛ مُهْجَة ؛ مِحْوَر ؛ نَفْس ؛ وَسَط ؛ وَلِيجَة ؛ وِجْدان
referenced from Arabic dictionary
we find there are main consciousness of the 'qof', 'ha' 'fa', ''suo' 'ja'
Hence, here the heart prophet mentioned might be align with consciousness of 'suo', whcih could be associated with 'suo' contiguousness in prayer 'salah', righteousness
The consciousness the ''qof' should be associated with 'qof' in word 'law' in arabic , stands for differentiation and integration.
The consciousness of the 'ja' of the heart might be associated with the word 'jooma' on Friday, leading to a purified condition. Hence 'ja' bear the double side, the best repayment is on the good trace of the heart. (between word jooma/ jannab)
The special heart that is the consciousness of the 'ha' and 'suo' together hence it might be associated the word of menstruation in arabic word, stands for the 'right of Adam's daughter' (righteousness of such nature 'Suo') but enclosed 'ha' as impurity.
Hence 'ha' might reflected on special consciousness towards heart, between the condition impurity under sort of righteousness. 'ha' might indicate reservation of its conditioned righteousness.
in this word, we find the 'heart' associated with menstruation, then related blood, and the ghusl.
it might stands for conditioned impurity under sort of natural righteousness, for reserved avoidance (menstruation avoid prayer) , and purify (ghusl) to be justified .
Hence we find in arbic word, the heart through words, associated with different natural and social affairs, then it might indicate why 'prophet did not be order to look into the heart', since it has been externalized through arabic language specifically might be.