The Prophet ﷺ used to say between the two prostrations, "O Allah, forgive me, have mercy on me, grant me well-being, guide me, and provide for me."
حدثنا محمد بن مسعود حدثنا زيد بن الحباب حدثنا كامل أبو العلاء حدثني حبيب بن أبي ثابت عن سعيد بن جبير
أن النبي ﷺ كان يقول بين السجدتين اللهم اغفر لي وارحمني وعافني واهدني وارزقني
[Hasan] Reported by Tirmidhi, Prayer, chapter on what is said between the two prostrations, no. 284, from the hadith of Zayd bin Hubab, transmitted through him. Ibn Majah also narrated it, no. 898. Hakim graded it sahih (1/262), and Dhahabi agreed with him. The core of the hadith has a corroborating narration in Muslim, no. 2697, see also no. 874, which is stronger than it. * Bin Abu Thabit is a mudallis and narrated it using an ambiguous form of transmission.