Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Allah Almighty had made it obligatory upon the Muslims that one man should face ten of the enemy. This was hard on them, so Allah lifted this ruling from them and made it that they should fight the enemy when the enemy is double their number.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11348