Is it obligatory to repeat the two rak'ahs for greeting the mosque after going to the mosque's restroom and returning to its center, considering that the restroom is part of the mosque?

Tahiyyat al-Masjid (greeting of the mosque prayer) is a Sunnah according to the majority of scholars, and it is not obligatory. Scholars differed on the ruling of sitting before praying, whether it is disliked or not.

As for repeating Tahiyyat al-Masjid for someone who frequently enters the mosque, there is a difference of opinion:

- The Hanafis, Malikis, and some Shafi'is are of the view that performing it once a day is sufficient. - The more authoritative view among the Shafi'is is that the greeting should be repeated whenever one enters, whether shortly after leaving or after a long time. - Some scholars believe that for someone who frequently enters the mosque, Tahiyyat al-Masjid is waived to remove hardship and difficulty. - Ibn 'Uthaymeen issued a fatwa that whoever leaves the mosque with the intention of returning soon should not pray Tahiyyat al-Masjid upon returning, as the first prayer suffices. - Ibn Baz issued a fatwa that whoever leaves and then returns should pray Tahiyyat al-Masjid along with the Sunnah of Wudu (ablution).

The matter is broad, and it is permissible for those who find it difficult to repeat it not to pray it.