Has it been reported from the Prophet (peace be upon him) the saying: "O Allah, protect us from Your torment on the Day You resurrect Your servants" before the opening takbeer (takbeerat al-ihram), and is the Imam's action of doing so in accordance with the Sunnah?

It has not been reported that the Prophet, peace and blessings be upon him, used to supplicate with the prayer "My Lord, protect me from Your punishment the day You resurrect Your servants" when straightening the rows. Rather, it has been reported that he used to supplicate with it when finishing the prayer, as indicated by the narrations of Muslim and Ahmad, and the narration of Ibn Khuzaymah which explicitly states that he said it "when he finished." Continually reciting this supplication when straightening the rows is not from the Sunnah and may lead to innovation (bid'ah).