Who was the first Muslim to migrate from Mecca to Medina?

The first of the supporters of the Messenger (saw) to migrate to Medina was the companion Abu Salama 'Abd Allah b. 'Abd al-Asad b. Hilal b. 'Abd Allah b. 'Umar b. Makhzum. The first of the emigrants to go there after Abu Salama was 'Amir b. Rabi'a, an ally of the Bana 'Adi. With him went his wife Layla, daughter of Abu Hathma al-'Adawiyya. Then followed 'Abd Allah b. Jahsh, an ally of the Banu Umayya , accompanied by his family and his brother 'Abd, that is Abu Ahmad. Abu Salama's migration took place one year before the pledge made at al-Aqaba.This was because, following his return from Abyssinia, he had been badly treated by Quraysh and had decided to return there, but, when he learned of fellow Muslims in Medina, he went there instead. Ibn Ishaq stated, "My father related to me, from Salama b. 'Abd Allah b. Umar b. Abu Salama, from his grandmother Umm Salama, who said, 'When Abu Salama decided to depart for Medina, he saddled his camel for me, mounted me on it and put my son Salama in my lap. He then led us away. When some men of the Banu al-Mughira saw him they approached and said, "We can accept what you yourself do, but why should we allow this woman of ours to be taken off by you somewhere else?" She went on, "And they snatched the camel's bridle out of his hand and took me off it." This angered the Banu 'Abd al-Asad, Abu Salama's people, and they said, "By God, we'll not leave a son of ours with her now you've taken her away from our man." So they tugged at my son Salama and dislocated his arm. The Banu 'Abd al-Asad then took him away, while the Banu al-Mughira kept me among themselves. My husband, Abu Salama, then left for Medina. So I was separated from both my son and my husband. Then one day a man from my uncle's family, one of the Banu al-Mughira, passed by, saw the state I was in and took pity on me. He told the Banu al-Mughira, "Can't you let this poor woman go? You've separated her from both her son and her husband!" They then told me I could join my husband if I liked. So then the Banu 'Abd al-Asad returned my son to me; I saddled a camel, took my son in my lap and headed out to my husband in Medina. I was entirely alone. Eventually, at al-Tan'im, I met Uthman bin Talha, a relative of the Banu 'Abd al-Dar, and he asked, "Where are you going, daughter of Abu Umayya?" I replied, "I'm heading for my husband in Medina." He said, "And there's no one with you?" "There is no one with me except God and this son of mine," I replied. He commented, "You shouldn't he left like that." He then took the camel's halter and began accompanying me. And, I swear, I was never in the company of any Arab more honourable than him. When we made a stop, he would make my camel kneel and then he would move away until I had dismounted. When I made a halt he would take my mount away, unload it and tie it to a tree. He would then move off and make his bed beneath a tree. When it was time to leave, he would get my camel, lead it up, saddle it and then stand away from me. He would then say, "Do mount!" And when I mounted and was secure, he would come and take its halter and lead me away until we next stopped He continued behaving in this way until he brought me to Medina. When he saw the village of the Banu Amr bin Awf at Quba, he said, "Your husband is in this village. Abu Salama is living here. Go on in, with God's blessings." He then left, returning to Mecca. She used to say, 'I know of no other family in all of Islam who suffered like that of Abu Salama. And I swear I never had a more honourable companion than Uthman bin Talha." (Ibn Kathir, al-Sira al-Nabawiyya, Vol. 2, pp. 144-145)