Dua for sitting between two prostrations:


"رَبِّ اغْفِرْ لِي رَبِّ اغْفِرْ لِي".
Rabbighfir lee, Rabbighfir lee.
Lord, forgive me. My Lord, forgive me.


Reference: Abu Dawud 1/231. See also AlAlbani, Sahihibn Mdjah 1/148.
It was narrated from Hudhaifah that:
He prayed with the Messenger of Allah (saw) one night and he heard him say when he said the takbir: "Allahu Akbara dhaljabaruti walmalakuti walkibriya'i wal'azamah (Allah is Most Great, the One Who has all power, sovereignty, magnificence and might.)" When bowing he would say: "Subhana Rabbial'Azim (Glory be to my Lord Almighty)." When he raised his head from bowing he would say: "Lirabbilhamd, Lirabbilhamd (To my Lord be praise, to my Lord be praise)." And when he prostrated (he said): "Subhana RabbialA'la (Glory be to my Lord Most High)." And between the two prostrations (he said): "Rabbighfirli, Rabbighfirli (Lord forgive me, Lord forgive me)." His standing, his bowing, when he raised his head from bowing, his prostration and the time between the two prostrations, were almost the same. (Sahih)

أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، قَالَ حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرِو بْنِ مُرَّةَ، عَنْ أَبِي حَمْزَةَ، عَنْ رَجُلٍ، مِنْ بَنِي عَبْسٍ عَنْ حُذَيْفَةَ، أَنَّهُ صَلَّى مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم ذَاتَ لَيْلَةٍ فَسَمِعَهُ حِينَ كَبَّرَ قَالَ ‏"‏ اللَّهُ أَكْبَرُ ذَا الْجَبَرُوتِ وَالْمَلَكُوتِ وَالْكِبْرِيَاءِ وَالْعَظَمَةِ ‏"‏ ‏.‏ وَكَانَ يَقُولُ فِي رُكُوعِهِ ‏"‏ سُبْحَانَ رَبِّيَ الْعَظِيمِ ‏"‏ ‏.‏ وَإِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ قَالَ ‏"‏ لِرَبِّي الْحَمْدُ لِرَبِّي الْحَمْدُ ‏"‏ ‏.‏ وَفِي سُجُودِهِ ‏"‏ سُبْحَانَ رَبِّيَ الأَعْلَى ‏"‏ ‏.‏ وَبَيْنَ السَّجْدَتَيْنِ ‏"‏ رَبِّ اغْفِرْ لِي رَبِّ اغْفِرْ لِي ‏"‏ ‏.‏ وَكَانَ قِيَامُهُ وَرُكُوعُهُ ‏.‏ وَإِذَا رَفَعَ رَأْسَهُ مِنَ الرُّكُوعِ وَسُجُودُهُ وَمَا بَيْنَ السَّجْدَتَيْنِ قَرِيبًا مِنَ السَّوَاءِ ‏.‏
Sunan anNasa'i 1069
Inbook reference : Book 12, Hadith 41
English translation : Vol. 2, Book 12, Hadith 1070