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.
Ibn Majah (897) narrated it through two paths, one of them from Ali bin Muhammad, who said, Hafs bin Ghiyath narrated to us, who said, Ala bin Musayab narrated to us, from Amr bin Mura, from Talha bin Yazid, from Huzaifa, who mentioned something similar. Hakim graded it sahih (1/271), meeting the standard of the two Shaykhs. Nasai (1664) also narrated it through the path of Ala bin Musayab, in a longer hadith, and said, "This hadith, in my view, is mursal, and I do not know that Talha bin Yazid heard anything from Huzaifa, and someone other than Ala bin Musayab said in this hadith, from Talha, from a man, from Huzaifa." End of quote.I say, perhaps he means by this Shuba, for he narrated it from Amr bin Mura, from Abu Hamza the freedman of the Ansar, from a man of Banu Abs, from Huzaifa. Abu Dawud (874) and Nasai (1145) both narrated it through a path from Shuba with this wording. In it, "He used to sit between the two prostrations for about as long as his prostration, and he used to say, 'My Lord, forgive me, my Lord, forgive me.'" The wording is Abu Dawud's.Abu Dawud Tayalisi (416) also narrated it from Shuba, and in it, "Then he raised his head from ruku, standing as long as his ruku, and said, 'To my Lord belongs all praise,' then he prostrated..." And he said, "He says between the two prostrations," mentioning something similar, and said, "Shuba believes that he is Salah bin Zafar, from Huzaifa." Bazzar (2935) said the same, that the man of Banu Abs, they believe, is Salah.I say, the hadith of Salah bin Zafar from Huzaifa was narrated by Muslim (772) at length, and it will come up in the chapter on night prayer. Part of it was already mentioned in the chapter on what is said in ruku and sujud, but it contains no mention of what is said between the two prostrations, except what Ibn Majah narrated through the second path, from Ali bin Muhammad, who said, Hafs bin Ghiyath narrated to us, from Amash, from Saad bin Ubayda, from Mustawrid bin Ahnaf, from Salah bin Zafar, from Huzaifa, that the Prophet ﷺ used to say between the two prostrations, "My Lord, forgive me, my Lord, forgive me."As for Shuba's own hadith, Abu Dawud Tayalisi (415) narrated it, and through his path Tirmidhi (262), Abu Dawud (871), and Nasai (1008) narrated it, both through routes from Shuba, from Amash, who said, I heard Saad bin Ubayda narrating from Mustawrid bin Ahnaf, from Salah bin Zafar, from Huzaifa, that he prayed with the Prophet ﷺ at night, and he would say in his ruku, "Glory be to my Lord, the Magnificent," and in his sujud, "Glory be to my Lord, the Most High," and whenever he came upon a verse of mercy, he would pause and ask, and whenever he came upon a verse of punishment, he would pause and seek refuge. This contains no mention of what is said between the two prostrations.So it appears, and Allah knows best, that the hadith has two paths, the path of Salah bin Zafar from Huzaifa, which is the well known one, and it contains no mention of what is said between the two prostrations. And the path of Ala bin Musayab, from Amr bin Mura, from Talha bin Yazid, from Huzaifa, which does contain mention of what is said between the two prostrations, and it is mursal, but it is supported by the hadith of Ibn Abbas that follows.