Some Muslim sources claim that after the conquest of northern Africa was completed by Muhammad ibn Jarir al-Tabari, Abdullah ibn Sa'd continued to Spain. Spain had first been invaded some sixty years earlier during the caliphate of Uthman. Other prominent Muslim historians, like Ibn Kathir, have quoted the same narration. In the description of this campaign, two of Abdullah ibn Saad's generals, Abdullah ibn Nafiah ibn Husain, and Abdullah ibn Nafi' ibn Abdul Qais, were ordered to invade the coastal areas of Spain by sea, aided by a Berber force. They succeeded in conquering the coastal areas of Al-Andalus. It is not known where the Muslim force landed, what resistance they met, and what parts of Spain they actually conquered. However, it is clear that the Muslims did conquer some portion of Spain during the caliphate of Uthman, presumably establishing colonies on its coast. On this occasion, Uthman is reported to have addressed a letter to the invading force: