The Prophet ﷺ used to say between the two prostrations, "My Lord, forgive me. My Lord, forgive me."
أن النبي ﷺ كان يقول بين السجدتين: رب اغفر لي، رب اغفر لي.
Narrated by Nasai (1665) and Ibn Majah (897), and Hakim graded it sahih (1/271), all of them through the route of Ala bin Musayab, from Amr bin Mura, from Talha bin Yazid Ansari, from Huzaifa, who related it. Nasai's version is longer. Nasai said, "In my view this hadith is mursal, and I do not know that Talha bin Yazid heard anything from Huzaifa, and narrators other than Ala bin Musayab said regarding this hadith, from Talha, from a man, from Huzaifa." End of quote.I say, it has been said that this unnamed man is Salah bin Zafar. The discussion of this is detailed at length in the book on prayer.