Is Tahiyat al-Masjid (greeting the mosque) obligatory for the preacher, or should he ascend the pulpit immediately upon entering the mosque? And is Tahiyat al-Masjid to be performed even if the Imam is delivering the sermon? And is it specific to Friday prayer or to all times?

The Sunnah is for the خطيب (preacher) to ascend the minbar directly without beginning with تحية المسجد (greeting the mosque prayer), as this is what the Prophet, peace and blessings be upon him, used to do. As for anyone other than the Imam, he must pray تحية المسجد before sitting down, whether it is during the Friday sermon or at any other time, unless the congregational prayer has already begun, based on the Prophet's, peace and blessings be upon him, saying: "If any of you enters the mosque, let him pray two rak'ahs before he sits down."