Prostration and its Excellence - Section 2 Mishkat al-Masabih 901
Arabic hadith text
وَعَنْ حُذَيْفَةَ أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَقُولُ بَيْنَ السَّجْدَتَيْنِ: «رَبِّ اغْفِرْ لي» . رَوَاهُ النَّسَائِيّ والدارمي
Translation
Hudhaifa reported that the Prophet used to say between the two sajdas, “My Lord, forgive me.” Nasa’i and Darimi transmitted it.