It is narrated from Ibn Abbas (may Allah be pleased with them) that he said: When this verse was revealed: "If there are twenty steadfast among you, they will overcome two hundred," it was made obligatory upon the Muslims that twenty men should not flee from facing two hundred. Then Allah the Exalted revealed: "Now Allah has lightened your burden, and He knows that there is weakness in you. So if there are a hundred steadfast among you, they will overcome two hundred." Thus, Allah lightened their burden and made it obligatory upon them that a hundred (Muslims) should not flee from facing two hundred (disbelievers).
Hadith Referenceمسند الإمام الشافعي / كتاب الجهاد وقسم الغنائم / 1729
Hadith Takhrijأخرجه البخاري، التفسير ، باب ﴿يايها النبى حرض المومنين على القتال﴾ (4652)۔