After suffering intense persecution from the pagan Meccans, the Prophet, may Allah bless him and grant him peace, instructed his companions - who had no tribal protection - to emigrate to Ethiopia (ard ul-Habasha).  He is reported to have said, “If you (all) would leave and go to Abyssinia (ard ul-habasha) (it would be better for you).  For verily, in it (i.e. Abyssinia) there is a king (malik) that does not do injustice to or oppress (la yadhlimu) anyone.  And it is a friendly country (ardun sidqun).  Therefore, seek protection with him until Allah brings about some relief from Himself for you.”

So, in the 5th year of his prophecy the companions, may Allah be pleased with them, left for Ethiopia and found a safe haven. As Ibn Ishaq said in his seerah, "With that, the Muslims from among the companions of the Messenger of Allah left to go to Abyssinia (ard ul-Habasha); fearing the persecution (al-fitnah) and fleeing to Allah with their way of life/Religion (bi deenihim).  This was the first migration (hijrah) in Al-Islaam."

Above is a copy of the original letter sent by the Prophet, may Allah bless him and grant him peace, to the king or Negus (an-Najashi in Arabic) years after the companions made hijrah there.  There's a difference of opinion regarding the scribe of this important letter.  Some of the 'ulama' say it was 'Ali ibn Abi Talib and others say it was 'Uthman ibn Affan who wrote the letter.

This highly overlooked contact between this African ruler and the early, pre-Medinan Muslim community marks the beginning of Al-Islam on the African continent.

Links

The letter from the Prophet Muhammad to an-Najashi - ruler of Ethiopia w/brief Commentary

Persecution which forced the hijrah (emigration)

Names of companions who made the hijrah

Ruling (hukm) on Hijrah

Verses revealed concerning the Cristianity of an-Najashi and his companions

History and Christianity of Ethiopia at the time of the Prophet, may Allah bless him and grant him peace