The Day of Yarmuk:

It was a momentous battle. The Byzantines descended upon the Yarmuk plain in Rajab of the year 15 AH—some say 13 AH, but that appears to be an error. Their numbers exceeded one hundred thousand, whereas the Muslims were thirty thousand, with Abū ʿUbaydah as overall commander and the chiefs of the various corps under him. The Byzantines had chained every five or six men together so that none might flee. When God put them to flight, a single soldier would plunge into the Yarmuk ravine and drag down with him all who were linked to his chain, until they had filled the valley and, it is said, made it level with both banks. The cavalry then trampled over them, and an innumerable multitude perished. Several Muslim commanders were martyred that day.

Muḥammad ibn Isḥāq relates: The Byzantines encamped at Yarmuk one hundred thousand strong, commanded by al-Saqlab, a eunuch in the service of Heraclius.

Ibn al-Kalbī says: The Byzantines numbered three hundred thousand, led by Mahan, a man of Banū Firās who had embraced Christianity and joined them. He adds that Abū ʿUbaydah drew in his outlying detachments, and ʿUmar reinforced him with Saʿīd ibn ʿĀmir ibn Ḥudhaym. God routed the polytheists after fierce fighting on the fifth of Rajab in the year 15 AH.

Saʿīd ibn ʿAbd al-ʿAzīz reports: The Muslims on the Day of Yarmuk were twenty-four thousand under Abū ʿUbaydah, while the Byzantines were one hundred and twenty thousand under Bāhān and Saqlab.