Ruling on saying Takbir after Taslim and before seeking Istighfar thrice

Q 4: Is it permissible for the one offering Salah (Prayer) to say, "Allahu Akbar (Allah is the Greatest)," after making Taslim (salutation of peace ending the Prayer) and before saying Istighfar (seeking forgiveness from Allah) three times? This is based on the word Takbir stated in the Hadith narrated by Ibn ‘Abbas (may Allah be pleased with him and his father) who said: “I used to recognize the completion of the Salah of the Prophet (peace be upon him) by hearing the Takbir (saying: "Allahu Akbar [Allah is the Greatest]").” (Agreed upon by Al-Bukhari and Muslim) If it is impermissible, what is meant by Takbir mentioned in the above Hadith?

A: When the Prophet (peace be upon him) concluded his Salah by offering Taslim, he (peace be upon him) used to start with Istighfar three times and then say: “Allahumma anta al-salam wa minka al-salam, tabarakta ya dhal-jalali wal-ikram (O Allah, You are Al-Salam [the One Who is free from all defects and deficiencies), and from You is all peace, blessed are You, O Possessor of Majesty and Honor).” Then he (peace be upon him) used to recite the other Adhkar (invocations) authentically reported. As for the Takbir mentioned in the Hadith, it means saying, "Subhan Allah (Glory be to Allah)", "Al-hamdu lillah (All praise is due to Allah)", and "Allahu Akbar (Allah is the Greatest)" thirty-three times after finishing every obligatory Salah. This is based on the Hadith narrated in this regard.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.