Sahih Bukhari Volume 5, Book 58, Hadith Number 216.

Narated By Abu Musa : We received the news of the departure of the Prophet (to Medina) while we were in Yemen. So we went on board a ship but our ship took us away to An-Najashi (the Negus) in Ethiopia. There we met Ja’far bin Abi Talib and stayed with him till we came (to Medina) by the time when the Prophet had conquered Khaibar. The Prophet said, “O you people of the ship! You will have (the reward of) two migrations.”