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."
أن النبي ﷺ كان يقول بين السجدتين: اللهم اغفر لي، وارحمني، وعافني، واهدني، وارزقني.
This was narrated by Abu Dawud (850), and this is his wording, and by Tirmidhi (284), and by Ibn Majah (898), all of them through the route of Kamil Abu Ala, who said, Habib bin Abu Thabit narrated to me, from Saeed bin Jubayr, from Ibn Abbas, and he mentioned something similar, except that Ibn Majah added in it, "in the night prayer," and Tirmidhi added, "and mend me," and he said, this is a gharib hadith. Some narrated this hadith from Kamil Abu Ala as a mursal narration. End of quote.I say, Kamil Abu Ala is a subject of disagreement. He was deemed reliable by Yahya bin Main, Ajli, and Yaqub bin Shayba, while others deemed him daif. The conclusion regarding him, as Hafiz said, is "truthful but makes mistakes."As for Habib bin Abu Thabit, although he was described as practicing tadlis, he is reliable in himself. He was deemed reliable by Yahya bin Main, Nasai, Ajli, and others. What was held against him was the hadith of the woman with prolonged bleeding, that she should pray even if blood drips onto the mat, and the hadith about kissing for the fasting person, because he did not hear the hadith of the woman with prolonged bleeding from Urwa, nor the hadith about kissing from Umm Salama, but rather transmitted both of them as mursal.Hakim graded it sahih (1/271) after narrating it through the route of Abu Ala, and said, "Kamil Abu Ala is among those whose hadith is gathered among the Kufans." This hadith was also narrated by Bayhaqi (2/122).As for Tirmidhi's statement, "narrated as mursal," I have not been able to identify who narrated it as such.