Is it obligatory for one who enters the mosque to stand and wait until the prayer is established if there isn't enough time to perform "Tahiyat Al-Masjid" (two rak'ahs upon entering the mosque)?

The Sunnah for someone entering a mosque is to pray Tahiyyat al-Masjid (greeting of the mosque), based on the Prophet's (PBUH) saying: "When any of you enters the mosque, he should not sit until he has prayed two rak'ahs." This is unless one enters Al-Masjid Al-Haram (the Sacred Mosque in Makkah), in which case performing Tawaf (circumambulation) is prescribed instead.

If a Muslim enters the mosque shortly before the Iqamah (call to commence prayer), there is a detailed ruling regarding Tahiyyat al-Masjid: - If he predominantly believes he will complete the two rak'ahs before the Takbirat al-Ihram (opening takbir for congregational prayer), he should pray them. - If he predominantly believes the Imam will say the Takbir while he is still praying, he should not pray Tahiyyat al-Masjid. Instead, he should remain standing, remembering Allah, until the prayer is established. This is so he does not miss the Takbirat al-Ihram with the Imam. - If the prayer is established while he is performing Tahiyyat al-Masjid, he should cut it short if he is in the first rak'ah. He should complete it if he is in the second rak'ah and can catch the ruk'u (bowing).