Imam An-Nawawi states in his Al-Adhkar, “…And between the two prostrations when one sits in Salah, one is to say the following supplication:
From the hadith of Hudhayfah: ” ربي اغفرلي  ربي اغفرلي  Rabbi Ighfirli! Rabbi Ighfirli! (My Lord forgive me, My Lord forgive me)” [Abu Dawud, An-Nasa’i, and Al-Bayhaqi]

And from the hasan (sound) hadith of Maymunah (may Allah be pleased with her) that she heard the Prophet (sallallahu ‘alayhi wa sallam) say, ”   ربي اغفرلي وارحمني واجبرني وارفعني وارزقني وهدني Rabbi ighfirli warHamni wajburni warfa’ni warzuqni wahdini” [Oh My Lord forgive me, have mercy on me, rectify me, uplift me, provide for me, and guide me!] Reported in Sunan Al-Bayhaqi and graded hasan by Imam An-Nawawi [page 80, Al-Adhkar, edit. Subay’ Hazimi].

And Allah Knows Best

Shafi’ifiqh.com Fatwa Dept.