Answered by Ustadh Tabraze Azam
Question: When praying in jama’a, and you finish your recitations/supplications before the Imam, what are the things you can say in that time?
Answer: Wa alaikum assalam wa rahmatullahi wa barakatuh,
I pray that you are well, insha’Allah.
If one finishes saying the tasbih of the bowing or the prostration before the Imam, one can either:  Recite additional tasbihs (keeping them ideally to an odd number), or  recite a short supplication.
`Aisha (Allah be pleased with her) relates that “the Prophet, may Allah bless him and grant him peace, used to say often in his bowing and prostration, ‘Glory be to You, O Allah, our Lord, and with Your praise. O Allah, forgive me! (Subhank Allahumma rabbana wa bi hamdik Allahumma ighfirli)’” [Agreed upon]
سُبْحَانَكَ الْلَّهُمَّ رَبَّنَا وَبِحَمْدِكَ اَللَّهُمَّ اغْفِرْلِي
If one finishes reciting in the final sitting before the Imam, one can recite further supplications until the Imam gives the closing salams. [Shurunbulali, Maraqi al-Falah]
And Allah alone gives success.
Checked & Approved by Faraz Rabbani