It is narrated from Sayyiduna Uqbah bin Amir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "May Allah have mercy on those who guard (the army)." He said this three times. Those who are between the Roman and Muslim armies will look after the Muslims and keep the disbelievers frightened.
Hadith Referenceمسند عمر بن عبد العزيز / عمر بن عبد العزيز عن أبيه / 83
Hadith Takhrijاسناده ضعيف: سنن ابن ماجه ، كتاب الجهاد ، رقم: 2769