Excerpts from "The Sealed Nectar"
When he returned to Mekkah, khadijah noticed greater profits and blessings in her business than usual. Her servant also told her of Muhammad's good manners, honesty, deep thought, sincerity and faith. Upon realizing this, she developed a wish for marriage. Earlier many prominent men had asked for her hand in marriage but she had always refused their advances. She disclosed her wish to her friend Nafisah, daughter of Muniyah, who immediately went to Muhammad Sallallahu Alaihi Wasallam to inform him.
He agreed and requested his uncles to go to Khadija's uncle to discuss the matter. Subsequently, they were married. The marriage agreement was witnessed by Bani Hashim and the cheats of Mudar. This took place after the Prophet's return from Syria. He gave her 20 camels as dowry. At the time she was 40 years old and was considered the best of women in lineage, fortune and wisdom. She was the first woman Allah's Messenger married. He did not get married to any other until she passed away.
With the exception of Ibrahim: Khadija bore all of his children Al-Qasim, Zainab, Ruqaiyah, Umm Kulthum, Fatimah and 'Abdullah who was called Taiyyib and Tahir. All his sons died in their childhood and all the daughters except Fathima died during his lifetime. Fatima died six months after his death. All his daughters witnessed Islam, embraced it, and emigrated to Madinah.