Sayyiduna Abu al-Darda' (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "The people of Sham, their wives, their children, their male slaves and their female slaves, up to the farthest limit of the Jazirah, will be counted as those guarding the frontier (murabitun). Whoever stays in any city of Sham is guarding the frontier, and whoever stays in one of the frontier outposts is a mujahid."
Al-Tabarani recorded it through Artat bin al-Mundhir, from a person who narrated it to him, from Sayyiduna Abu al-Darda' (may Allah be pleased with him); he did not name that person, and the rest of its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 16661