The letter which Umar wrote to Abu Ubaidah, appointing him the commander of the army, reached him while a great battle was in progress, and Abu Ubaidah withheld this information from Khalid until after the battle had been won. This battle could not have been that of Ajnadein, which was fought-and on this there is universal agreement-in Jamadi-ul-Awwal, 13 Hijri. Some writers have quoted sources saying that the event took place at Yaqusa. But this could not be correct either, as Yaqusa was just a one-day battle. Some who consider that the year of the Battle of Yarmuk was 13 Hijri have said that the letter reached Abu Ubaidah during this battle. This too is incorrect, as the Battle of Yarmuk was fought in 15 Hijri.