Answer
(Fatwa: 831/694/B=1432)
According to Hanafi Fiqh, saying “رب اغفرلي” between two sajdah is mustahab while offering Fardh, sunnah or nafl alone. Also a muqtadi following an imam in fardh salah may read it provided he is following the imam in all arkan (practices) with reading the dua. However, an imam should not develop this habit, because he has been asked in hadith to shorten his salah. However, it is not wajib to say “رب اغفرلي” according to Hanafi Fiqh, neither for individuals nor for imam and muqtadi. (Rad al-Muhtar, Imdadul Fatawa, Vol. 1, p. 200)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband