First he swept down on the Bani Hanifa in Yemama, who with their rival prophet Mosailama (Mosailima) and 40,000 men were in arms. The battle of Yemama (633) was fierce and decisive.
He encountered formidable opposition from different quarters, but in every case he was successful, the severest struggle being that with the impostor Mosailima, who was finally defeated by Khalid at the battle of Akraba.
After the hard-won victory over Mosailima, Omar, fearing that the sayings of the prophet would be entirely forgotten when those who had listened to them had all been removed by death, induced Abu-Bekr to see to their preservation in a written form.
IIanifa under their prophet Mosailima fought bravely, but here also Islam triumphed.