I saw most of the Muslims who have completed their Hajj add the prefix Al-Haj
or Haji
to their names, like adding Al-Hafiz
to those who finished memorizing the whole Al-Quran.
Haji is an honorific title given to a Muslim person who has successfully completed the Hajj to Mecca. Women who complete the Hajj are referred to as Hajiya - Wikipedia
Is it a sunnah or bid'ah?