If one says, "My Lord, forgive me," instead of "Sami'a Allahu liman hamidah (Allah hears those who praise Him)," while rising from bowing (ruku'), and then immediately corrects it by saying "Sami'a Allahu liman hamidah" after rising, does this necessitate two prostrations of forgetfulness (sujood al-sahw) before the salutation (tasleem)?

The questioner's action is correct, and it is the prostration of forgetfulness (sujood as-sahw), because he recited a prescribed supplication ("Rabbighfir li - My Lord, forgive me") in an improper place, which was when rising from bowing (ruku'). According to Hanbali jurists, it is Sunnah to perform the prostration of forgetfulness for someone who recites a prescribed supplication in an improper place, and its place is before the salutation (taslim).