Muhammad Mustafa (pbuh), shared all the sorrows and afflictions of his followers who were being persecuted for believing that "God is One", but he had no means to protect them. When the violence of the polytheists against the Muslims didn't show any sign of de-escalating, he suggested to them to leave Makkah and to seek sanctuary in Abyssinia (Ethiopia) which was then ruled by a Christian king, well-known for being a just and God-fearing man. Following this suggestion, a group of Muslims, comprising eleven men and four women, left Makkah and went to Abyssinia. The group included Uthman bin Affan, a future khalifa of the Muslims; his wife, Ruqayya; and Zubayr bin al-Awwam, a cousin of the Prophet. The Prophet appointed Uthman bin Mazoon, one of his principal companions, as the leader of this group.
Ibn Ishaq

When the Apostle saw the afflictions of his companions and that though he escaped it because of his standing with Allah and his uncle, Abu Talib, he could not protect them, he said to them: ‘If you were to go to Abyssinia (it would be better for you), for the king (there) will not tolerate injustice and it is a friendly country, until such time as Allah shall relieve you from your distress.' Thereupon his companions went to Abyssinia, being afraid of apostasy and fleeing to God with their religion. This was the first hijra in Islam. (The Life of the Messenger of God)

The first migration took place in the fifth year of the Proclamation – in A.D. 615.

The king of Abyssinia welcomed the Muslim refugees from Makkah into his kingdom. He gave them sanctuary, and they enjoyed peace, security and freedom of worship under his aegis. About a year later, the Muslims in Abyssinia heard rumors that the Quraysh in Makkah had accepted Islam. If it was true then there was no reason for them to live in exile. They were homesick, and they decided to return to Makkah. But when they arrived in Makkah, they found out that not only the rumors they had heard were false, but also that the Quraysh had stepped up the persecution of the Muslims. They, therefore, left Makkah once again. Many other Muslims also accompanied them. This new group comprised 83 men and 18 women. Muhammad Mustafa appointed his first cousin, Jaafer ibn Abi Talib, an elder brother of Ali, as the leader of this group.

This second migration of the Muslims to Abyssinia took place in the sixth year of the Proclamation, which corresponds to the year A.D. 616.

The migration of the Muslims to Abyssinia, and their reception at the friendly court of that country, alarmed the Quraysh. They entertained the fear that Muslims might grow in strength, or find new allies, and then, some day, might return to Makkah to challenge them. To head off this potential threat, such as they saw it, they decided to send an embassy to the court of the king of Abyssinia to try to persuade him to extradite the Muslims to Makkah.

The Muslim refugees who had expected to be left in peace, were surprised by the arrival, in the Abyssinian capital, of an embassy from Makkah, led by a certain Amr bin Aas. Amr had brought rich presents for the king and his courtiers to ingratiate himself with them.

When the king gave audience to the emissary of the Quraysh, he said that the Muslims in Abyssinia were not refugees from persecution but were fugitives from justice and law, and requested him to extradite them to Makkah. The king, however, wanted to hear the other side of the story also before giving any judgment, and summoned Jaafer ibn Abi Talib to the court to answer the charges against the Muslims.

Jaafer made a most memorable defense. Following is a summary of his speech in the court of Abyssinia in answer to the questions posed by the Christian king.

"O King! We were ignorant people and we lived like wild animals. The strong among us lived by preying upon the weak. We obeyed no law and we acknowledged no authority save that of brute force. We worshipped idols made of stone or wood, and we knew nothing of human dignity. And then God, in His Mercy, sent to us His Messenger who was himself one of us. We knew about his truthfulness and his integrity. His character was exemplary, and he was the most well-born of the Arabs. He invited us toward the worship of One God, and he forbade us to worship idols. He exhorted us to tell the truth, and to protect the weak, the poor, the humble, the widows and the orphans. He ordered us to show respect to women, and never to slander them. We obeyed him and followed his teachings. Most of the people in our country are still polytheists, and they resented our conversion to the new faith which is called Islam. They began to persecute us and it was in order to escape from persecution by them that we sought and found sanctuary in your kingdom."

When Jaaffer concluded his speech, the king asked him to read some verses which were revealed to the Prophet of the Muslims. Jaafer read a few verses from Surah Maryam (Mary), the 19th chapter of Al-Qur’an al-Majid. When the king heard these verses, he said that their fountainhead was the same as that of the verses of the Evangel. He then declared that he was convinced of his veracity, and added, to the great chagrin of Amr bin Aas, that the Muslims were free to live in his kingdom for as long as they wished.

But Amr bin Aas bethought himself of a new stratagem, which, he felt confident, would tilt the scales against Jaafer. On the following day, therefore, he returned to the court and said to the king that he (the king) ought to waive his protection of the Muslims because they rejected the divine nature of Christ, and claimed that he was a mortal like other men. When questioned on this point by the king, Jaafer said:

"Our judgment of Jesus is the same as that of Allah and His Messenger, viz., Jesus is God's servant, His Prophet, His Spirit, and His command given unto Mary, the innocent virgin."

The king said:

"Jesus is just what you have stated him to be, and is nothing more than that."

Then addressing the Muslims, he said:

"Go to your homes and live in peace. I shall never give you up to your enemies."

He refused to extradite the Muslims, returned the presents which Amr bin Aas had brought, and dismissed his embassy.

Washington Irving

Among the refugees to Abyssinia, there was Jaafer, the son of Abu Talib, and brother of Ali, consequently the cousin of Mohammed. He was a man of persuasive eloquence and a most prepossessing appearance. He stood forth before the king of Abyssinia, and expounded the doctrines of Islam with zeal and power. The king who was a Nestorian Christian, found these doctrines so similar in many respects to those of his sect and so opposed to the gross idolatry of the Koreishites, that so far from giving up the fugitives, he took them more especially into favor and protection, and returning to Amr b. Aas and Abdullah, the presents they had brought, dismissed them from his court. (Life of Mohammed)

Muslims spent many years in Abyssinia and lived there in peace. Thirteen years later – in 7 A.H. (A.D. 628) – they returned, not to Makkah but to Medina. Their arrival synchronized with the conquest of Khyber by the Muslims.

Jaafer ibn Abi Talib was the leader of all those Muslims who had migrated to Abyssinia in 615 and 616. He appears to have been the only member of the clan of Banu Hashim to leave for Abyssinia with the other refugees. All other members of Banu Hashim stayed in Makkah.

Montgomery Watt