أَعْلَمُ (Allah is most knowing): It is mentioned in the Holy Qur’an in many verses. For it is a superlative adjective. i.e. more aware than you of the reality of things and their interior facts.

يُوعُونَ (what they keep within themselves): The Arabic verb is derived from the noun (Wea’a) meaning a vessel that is a means of including things, whether material, such as drink or clothes as in this verse:

{ فَبَدَأَ بِأَوْعِيَتِهِمْ قَبْلَ وِعَاءِ أَخِيهِ}

(So he began [the search] with their bags before the bag of his brother;) (Yusuf: 76)

or immaterial, that is, the heart is a vessel of emotions, intentions, and consciences. So, Allah (swt) is the most knowing of what they keep in their hearts.