9. "Our Lord, we have believed, so register us among the witnesses. And why should we not believe in Allah and what has come to us of the truth? And we aspire that our Lord will admit us [to Paradise] with the righteous people." [5:83 and 84]

Whose supplication is this?

Explanation:

Allah Subhanahuthala addressed to the people of the Book (that is Jews and Christians) in Surah - Al - Maidha and warned them a lot about what they are doing.

Allah Suhanahuthala said:

"You will surely find the most intense of the people in animosity toward the believers [to be] the Jews and those who associate others with Allah ; and you will find the nearest of them in affection to the believers those who say, "We are Christians." That is because among them are priests and monks and because they are not arrogant." [5:82]

And when they hear what has been revealed to the Messenger, you see their eyes overflowing with tears because of what they have recognized of the truth. They say, "Our Lord, we have believed, so register us among the witnesses. [5: 83]

And why should we not believe in Allah and what has come to us of the truth? And we aspire that our Lord will admit us [to Paradise] with the righteous people." [5:84]

To believing people from the people book Allah Subhanahuthala says:

Surely those who believe, and those who are Jews, and the Christians, and the Sabians—whoever believes in Allah and the Last Day and does good, they shall have their reward from their Lord. And there will be no fear for them, nor shall they grieve" (2:62)

Indeed, those who have believed [in Prophet Muḥammad (SAW)] and those  who were Jews or Sabeans or Christians - those [among them] who believed in Allāh and the Last Day and did righteousness - no fear will there be concerning them, nor will they grieve. [5:69].

الحمدلله