Reference: http://www.alfawzan.af.org.sa/node/14749
Question: What is required to be said during the prostration of forgetfulness?
Response: [One should utter] similar to what is [uttered] during the [regular] prostration of the prayer – [that being] Subhaana Rabbee al-A’laa [سُبْحَانَ رَبِّي الأَعْلَى]; and this is to be repeated [three times], and supplicate [to Allaah] if he so wishes.