It was a custom of the Quraysh nobility to send their newborns away from Makkah to be raised by desert wet-nurses: to protect them from the diseases of crowded settlements, to harden them to a difficult life, to spare them the pampering of relatives, and, above all, because certain desert tribes were famed for speaking the purest, most fluent Arabic, uncorrupted by the loan-words of the city. The most famous such tribe was Banu Sa'd ibn Bakr, and it was they who fostered the Prophet ﷺ. Every year the women of Banu Sa'd would come to Makkah to take in the infants of the Quraysh in exchange for payment. Halimah al-Sa'diyah and her husband were suffering great poverty; her own newborn son had just caused her milk to flow again, so she came to Makkah that year to find a child to nurse for two or three years. Everyone else in her group found an infant to foster except her: the only child left unclaimed was the orphan whose father had died, since the other women expected little payment from a fatherless child's family. With every other woman already provided for, Halimah agreed to take him, and the household experienced immediate blessings: an old goat that had stopped giving milk began producing again, and an aging mount became the fastest of the group. The blessings Halimah witnessed convinced her to extend the arrangement beyond the usual two years, and Aminah bint Wahab agreed even though she would receive no additional payment.
While the Prophet ﷺ, then four years old, was playing with the other children of the household, Jibril came to him. The other children fled in fear, but he stood his ground. Jibril overpowered him, laid him down, and opened his chest; he took out his heart, removed a black portion from it ("shaytan's portion") and discarded it, then washed the heart in a golden vessel of Zamzam water and sealed the chest back up. Halimah al-Sa'diyah's son and her daughter Shayma ran back to her screaming that their brother had been killed; when Halimah reached him she found him sitting quietly with a pale face, showing fear but not crying out. A visible line remained on his chest ﷺ for the rest of his life. Shaken by the incident, Halimah decided to return him to his mother, Aminah.
After the tribes had entered Islam, Banu Sa'd ibn Bakr was fought and defeated at the Battle of Hunayn. A woman of the tribe caused a commotion, claiming to be the Prophet ﷺ's sister, and the companions brought her before him. This took place among the prisoners of war being led in to Ji'ranah after the battle. Shayma cried out that she was the Prophet ﷺ's own foster sister, daughter of his wet-nurse Halimah al-Sa'diyah, and the astonished Companions brought her to him.
Halimah al-Sa'diyah and her husband had not been present at Hunayn, and only came to visit afterward. When the Prophet ﷺ saw her, he recognized her instantly, stood to greet her, and took off his own shawl to spread on the ground for her to sit on.
The Banu Sa'd ibn Bakr were the people of Halimah, who had nursed the Prophet ﷺ in the desert as an infant, and a branch of the Hawazin who had fought him at Hunayn. Bedouins rather than townsmen like the Thaqif, they were rough in their manners, and the man they sent to Madinah in the ninth year to negotiate a treaty was not yet a Muslim. Dimam ibn Tha'laba, an elder of the tribe, hairy and unkempt, his hair in two plaits, rode his camel to the door of the mosque, couched it there, walked in and demanded to know which of them was the son of Abdul Muttalib. Ignorant and unlettered as he was, he knew the lineage, and that mattered to a man for whom lineage was everything. The Prophet ﷺ said that he was the son of Abdul Muttalib. You are Muhammad ﷺ? I am. I am going to ask you, Dimam said, and I am going to be hard with you, so do not be angry with me. Go on. Your envoy came to us and said that Allah has sent you. He spoke the truth. Who created the heavens? Allah. Who created the earth? Allah. Who set up the mountains? Allah. Who put everything in its place? Allah. Then I ask you by the One who created the heavens and the earth and set up the mountains: do you swear that Allah has sent you to us? I swear by Allah that Allah has sent me. Your envoy said we must pray five times a day; by the One who sent you, did Allah command you to tell us this? Yes. And so on through zakat, fasting and the pilgrimage, the last of which fixes the year, for Hajj had only just been made obligatory. Some say he is the man of the well-known hadith who then asked whether he owed anything beyond the five prayers, the zakat and the fast of Ramadan, and being told no, unless he wished, swore by the One who had sent him with the truth that he would do that much and neither add to it nor take from it. When he had gone the Prophet ﷺ said to the Companions: if he is true, he will enter Paradise. He went home and preached to his people, and Ibn Ishaq says that such was his standing that the whole tribe accepted Islam the day he returned, and that same day he broke the idol they had worshipped. No delegate, it was said, was a greater blessing to his people than Dimam.