How sound is the supplication: "When a servant repents, the angels say: O Allah, gladden him as You have gladdened us, and they say: As long as he has brought us joy and repented, then O Lord! Gladden him, and Allah gladdens His servant with Paradise"?

There is no basis for the statement of the angels upon a servant's repentance: "O Allah, make him rejoice as we have rejoiced." None of the scholars have mentioned it. It was mentioned by a contemporary preacher without any chain of narration. What is established in the Sunnah is that Allah rejoices at the repentance of His servant, and the angels also rejoice at that, and they seek forgiveness for the believing repentant, as mentioned in Surah Ghafir, verse 7.