قُلْ لَوْ كَانَ فِي الْأَرْضِ مَلَائِكَةٌ يَمْشُونَ مُطْمَئِنِّينَ لَنَزَّلْنَا عَلَيْهِمْ مِنَ السَّمَاءِ مَلَكًا رَسُولًا ﴿۹۵﴾ قُلْ كَفَى بِاللَّهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ إِنَّهُ كَانَ بِعِبَادِهِ خَبِيرًا بَصِيرًا ﴿۹۶﴾

﴾95﴿ Qul law kaana fil ardi malaaa 'ikatuny yamshoona mutma'in neena lanazzalnaa 'alaihim minas samaaa'i malakar Rasoolaa
﴾96﴿ Qul kafaa billaahi shaheedam bainee wa bainakum; innahoo kaana bi'ibaadihee Khabeeram Baseeraa

﴾95﴿ Say, If there were angels walking and living peacefully on the earth, We would have sent down to them an angel as a messenger from the heavens
﴾96﴿ Say, Sufficient is Allah as a witness between me and you. Indeed, Allah is ever All-Knowing, All-Seeing of His servants

[95] This verse is the answer to the previous question. The essence of the answer is that the messenger must be of the same kind as those to whom he is sent. If there were angels living on earth, then certainly their messenger would also be an angel of their kind. But since you are human, your messenger must necessarily be human as well.
The phrase "يَمْشُونَ مُطْمَئِنِّينَ" (walking in peace and stability) alludes to the idea that if angels were such that they lived on earth and could not ascend to the heavens, then their way of life would resemble that of humans. This was stated to clarify that although angels do sometimes descend to earth, they travel back and forth to the heavens.
[96] This too is an answer and a confirmation of the Prophethood of the Messenger of Allah (May Allah bless him and give him peace), established by the testimony of Allah the Exalted. A similar statement appears in verse 43 of Surah Ar-Ra'd.
The word "شَهِيدًا" (as a witness) refers to Allah’s testimony to the Prophethood of the Messenger of Allah (May Allah bless him and give him peace), which consists of the reports, evidences, and arguments that Allah has mentioned in many verses of the Noble Qur'an. These serve as decisive proofs between the two groups (believers and deniers), which is why the phrase "بيني و بينكم" (between me and you) is used.
Or, the meaning of this testimony could be Allah's knowledge of the state of the Prophet and the deniers—meaning that if, hypothetically, this Prophet were not true, Allah the Exalted would have destroyed him, as mentioned in verses 45 and 46 of Surah Al-Haqqah.