Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.156.

It is preferred to make one of the following supplications between the two prostrations. One may repeat them more than once if one wishes to do so. An-Nasa’i and Ibn Majah recorded that Huzhaifah reported that between the two prostrations, the Prophet would say, “O Lord, forgive me.” Abu Dawud recorded from Ibn ‘Abbas that while prostrating, the Prophet, upon whom be peace, would say, “O Allah, forgive me, have mercy on me, grant me well-being, guide me and provide for me.”