Praise be to Allaah.
This hadeeth is one of the saheeh hadeeths that speak of the virtues of Greater Syria (Bilaad al-Shaam). It was narrated by the great Sahaabi Zayd ibn Thaabit (may Allah be pleased with him) who said: We were with the Messenger of Allah (blessings and peace of Allah be upon him), collecting the Qur'aan from pieces of vellum, and the Messenger of Allah (blessings and peace of Allah be upon him) said: Blessed is (Greater) Syria. We said: Why is that, O Messenger of Allah? He said: Because the angels of the Most Merciful spread their wings over it.
Narrated by al-Tirmidhi (no. 3954); he said: Hasan ghareeb. Also narrated by Imam Ahmad in al-Musnad (35/483, Mu'sasat al-Risaalah edn.), classed as saheeh by the commentators and by Shaykh al-Albaani in al-Silsilah al-Saheehah (no. 503).
al-Mubaarakfoori (may Allah have mercy on him) says:
Blessed is (Greater) Syria means: because it and its people have a good life.
Because the angels of the Most Merciful this refers to the angels of mercy.
spread their wings over it means: over the region of (Greater) Syria and its people, protecting them from kufr. This was stated by al-Qaari. Al-Manaawi said: i.e., surrounding it, sending down blessings and warding off calamity and harm. End quote.
Tuhfat al-Ahwadhi, 10/315.
And Allah knows best.