Explanation of the Book of Prayer-15

The Shaykh :" And from Abu Qatadah (may Allah be pleased with him), who said: The Messenger of Allah (peace and blessings be upon him) said:

If anyone of you enters a Mosque, he should not sit until he has offered a tworak` at prayer.

إذا دخل أحدكم المسجد فلا يجلس حتى يصلي ركعتين

Sahih al-Bukhari 1167

. ".
His saying the mosque is the place prepared for prayer in which prayer is established, and it is not intended to mean every place of prayer, but rather the customary mosques that are open for people to pray in.
And "al" in the phrase "the mosque" indicates comprehensiveness, meaning any mosque you enter, whether it is small or large, a congregational mosque or one for the five daily prayers.
meaning in the mosque if he intends to sit down.
And these two rak'ahs are called by the scholars the greeting of the mosque.