Surah Al Mu'minun Tafseer

And those who give that which they give with their hearts full of fear, because they are sure to return to their Lord.

means, they give in charity, fearing that it may not be accepted from them because of some shortcoming or failure to meet the required conditions of giving.

This has to do with fear and taking precautions, as Imam Ahmad recorded from A'ishah:

I said:

"O Messenger of Allah,

وَالَّذِينَ يُوْتُونَ مَا اتَوا وَّقُلُوبُهُمْ وَجِلَةٌ

(And those who give that which they give with their hearts full of fear...),

Are these the ones who steal and commit Zina and drink alcohol while fearing Allah!"

The Messenger of Allah replied:

لَاا يَا بِنْتَ أَبِي بَكْرٍ يَا بِنْتَ الصِّدِّيقِ وَلَكِنَّهُ الَّذِي يُصَلِّي وَيَصُومُ وَيَتَصَدَّقُ وَهُوَ يَخَافُ اللهَ عَزَّ وَجَل

No, O daughter of Abu Bakr. O daughter of As-Siddiq, the one who prays, fast and gives in charity, fearing Allah.

This was recorded by At-Tirmidhi, and Ibn Abi Hatim recorded something similar in which the Prophet said:

لَاا يَا بِنْتَ الصِّدِّيقِ وَلَكِنَّهُمُ الَّذِينَ يُصَلُّونَ وَيَصُومُونَ وَيَتَصَدَّقُونَ وَهُمْ يَخَافُونَ أَلاَّ يُقْبَلَ مِنْهُمْ

No, O daughter of As-Siddiq, they are the ones who pray and fast and give in charity while fearing that it will not be accepted from them