Is it true that the Messenger of Allah, may Allah bless him and grant him peace, used to pray "Salat al-Khamsin" after 'Isha, and if so, how is it performed?

There is no prayer called "Salat al-Khamsin" (the Fifty Prayer), and the statement that the Prophet, peace and blessings be upon him, used to pray it is incorrect, as evidenced by the fact that he did not exceed eleven or thirteen rak'ahs in his night prayer.