What is your evidence that the worshiper's saying, "Lord, forgive me, Lord, forgive me" is an obligation and not a Sunnah?

The supplication between the two prostrations is a confirmed Sunnah from the Prophet, peace and blessings be upon him. Its minimum is: "My Lord, forgive me," and its most complete form consists of seven words: "O Allah, forgive me, and have mercy on me, and mend my affairs, and guide me, and provide for me, and grant me well-being, and raise my status." Scholars have differed regarding its ruling. The majority of them hold that it is recommended, while the Hanbalis consider it obligatory. However, the minimum obligatory according to them is "My Lord, forgive me" once, and any addition is recommended. The difference between an obligatory act (wajib) and a fard (an absolute obligation) is that a wajib is waived by forgetfulness and compensated by the prostration of forgetfulness (sujud al-sahw), whereas a fard is not waived whether intentionally or by forgetfulness.