In Jumada al-Awwal of 6 AH, the Prophet ﷺ sent Zayd ibn Harithah RA with 170 men to al-'Is, north of Madinah. Their task was to intercept a caravan of the Quraysh returning from Syria. They captured the whole caravan, with its camels, its goods and its men. One of the prisoners was Abu al-As ibn al-Rabi', the Prophet ﷺ's son-in-law and the husband of his daughter Zaynab. Abu al-As had already been taken prisoner once before, at Badr.
The next morning, as the Prophet ﷺ began the Fajr prayer, Zaynab called out from the women's section. She announced that she had given Abu al-As her protection, and asked the Muslims to give him theirs as well. After the prayer, the Prophet ﷺ told the people that he had known nothing of this beforehand. He then declared that any Muslim, even the least of them, can grant protection, and that the Muslims would protect whomever Zaynab had protected.
The Prophet ﷺ then spoke to the men who had captured Abu al-As's part of the caravan. He told them they could return it to him if they wished, and that this would be more beloved to him, but that keeping it was their right. The Ansar who held it agreed to give it back. First, though, they urged Abu al-As to accept Islam, saying he could then keep the wealth and stay in Madinah. Abu al-As refused. He said it would be wrong to change his faith and betray a trust for the sake of money. They gave the wealth back to him anyway.
The wealth was not his own but the investments of the people of Makkah. Abu al-As took it back to Makkah and returned every last part of it to its owners. Once all his obligations were settled, he declared the shahada in front of the Quraysh, so that no one could say he had become Muslim for money or to save his life. He then returned to Madinah as a Muslim and was reunited with Zaynab.