Hz Khadijah (RA) was the first wife of our beloved Prophet Muhammad (PBUH). She was the first person to believe in the Prophethood of Prophet Muhammad (PBUH).
Once Hz Aisha (RA) asked Prophet Muhammad (PBUH) whether Hz Khadijah (RA) was the only woman worthy of his love. He replied:
She believed in me when no one else did; she accepted me when others rejected me; and she helped and comforted me when there was no one else to lend me a helping hand.
Inspiring Muslim Women: Khadijah bint Khuwaylid (RA)
Hz Khadijah bint Khuwaylid (RA), born in the year 556 CE in Mecca, was the daughter of a leader of Quraysh, Khuwaylid bin Asad. Her father was a popular businessman who unfortunately died in a battle when Hz Khadijah (RA) was just a child. After him, she too became a successful businesswoman. Not only was she successful in business, she was also renowned for her good character, kindness and generosity. In fact, on account of her piety, she was often known as “Tahira”, meaning “the pure one”.
Before marrying Prophet Muhammad (PBUH), she was widowed twice. She was so overwhelmed with Prophet Muhammad (PBUH)’s character that she sent her nephew Qatima to find out if Prophet Muhammad (PBUH) would accept a post as her trade agent. Prophet Muhammad (PBUH) consulted his uncle Abu Talib and accepted Khadijah (RA)’s proposition. She arranged a reasonable salary, and Prophet Muhammad (PBUH) set out on his journey to Syria, accompanied by Hz Khadijah (RA)’s slave Maysarah and one of her relatives, Khazima ibn Hakeem. The trade mission to Syria proved so profitable that Khadijah (RA) was filled with admiration for Prophet Muhammad (PBUH) .
Trustworthiness, honesty and truthfulness of Prophet Muhammad (PBUH) was praised by every tribe and individual in Mecca. His dignified character and sincerity had won the heart of Khadijah bint Khuwaylid (RA). Many noblemen of the Quraysh desired to marry her, but she declined because she had developed great affection and respect for Prophet Muhammad (PBUH) on account of his character. Hz Khadijah (RA) sent a marriage proposal to Prophet Muhammad (PBUH). Prophet Muhammad (PBUH) consulted his uncle Abu Talib, and then accepted the proposal. Hz Khadijah (RA) was forty years old at that time, and Prophet Muhammad (PBUH) was twenty-five.
Prophet Muhammad (PBUH) and Hz Khadijah (RA) had two sons, Qasim and Abdullah, both of whom passed away early in childhood. They had four daughters, Zaynab (RA), Umme-Kulthum (RA), Ruqayyah (RA) and Fatimah (RA).
Hz Aisha (RA) reported that Allah’s Prophet (PBUH) did not marry any other woman until her (Khadijah’s) death. 
Prophet Muhammad (PBUH) often used to seclude himself in a cave in Mount Hira, a few miles from Mecca. It was during one of those retirements that Archangel Gabriel came to him to tell him that he was the Messenger of Allah and gave him the Revelation from Allah and asked him to read: 
Read, in the name of Allah who created (everything). He created man from a clot of blood. Read, your Lord is Great who taught you by virtue of the pen that which you did not know.
Prophet Muhammad (PBUH) repeated the words with a trembling heart. He returned to Hz Khadijah (RA) from Mount Hira and said: “Wrap me up! Wrap me up!” She wrapped him in a garment until his fear was dispelled. He told Hz Khadijah (RA) what had occurred. She replied: “Allah forbid! He will surely not let a wrong thing happen, for you speak the truth, you are faithful in trust, you bear the afflictions of the people, you spend in good works what you gain in trade, you are hospitable and you assist your fellow-men. Have you seen anything terrible?” Prophet Muhammad (PBUH) replied: “Yes,” and told her what he had seen. Whereupon, Hz Khadijah (RA) said: “Rejoice, O dear husband and be cheerful. He in Whose Hands stands Khadijah (RA)’s life bears witness to the truth of this fact, that you will be the Prophet to this people.”
Then she went to her cousin Waraqa ibn Naufal, who knew the Scriptures of the Jews and Christians, and is stated to have translated them into Arabic. When she told him of what she had heard, he cried out: “Holy! Holy! Just as the Archangel Gabriel had come to Moses (AS) before and had ordered him to guide his people, so, too, would Muhammad (PBUH) be the Prophet of his people.”
But Waraqah warned that all the people would not listen to the Prophet (PBUH) and some would mistreat his followers. He must, however, be patient because he had a great message for all the world. he said “I swear by Him in Whose Hand Waraqa’s life is, Allah has chosen you to be the Prophet of this people. They will call you a liar, they will persecute you, they will banish you, and they will fight against you. Oh, that I could live to those days. I would fight for these.”
Hz Khadijah (RA) was the first one to embrace Islam. With time, the spread of Islam among the tribes aggravated the anger of the Quraysh. They came together and decided to undertake social and economic boycott of Muslims. This boycott was put into writing and the parchment was hung in the Ka’bah in order to give it a religious sanction thereby making it mandatory for all.
Thus, for three long years, Muslims suffered severe hardship and starvation, plus want of clothing and bare necessities. Hz Khadijah (RA), the richest lady of Mecca, bore these hardships and sacrificed her wealth in the cause of Islam.
Hz Aisha (RA) reported that Allah’s Messenger (PBUH) gave glad tidings to Hz Khadijah bint Khuwaylid (RA) of a palace in Paradise. 
Soon after the end of the boycott, in the tenth year of his Prophethood, the Prophet (PBUH) lost his loving wife Hz Khadijah (RA) as well as his uncle Abu Talib; as such, this year would later be known as ‘The Year of Sorrow’.
For the rest of his life, Prophet Muhammad (PBUH) would often remember his most encouraging companion, Hz Khadijah (RA). Narrated by Hz Aisha (RA): 
I did not feel jealous of any woman as much as I did of Khadijah (RA) because Allah’s Apostle (PBUH) used to mention her very often.
Hz Khadijah (RA) died at the age of 65 years, nearly three years before Hijrah. This was indeed an irreparable loss! Narrated by Hz Ali (RA): 
The Prophet (PBUH) said, “The best of the world’s women is Mary (at her lifetime), and the best of the world’s women is Khadijah (at her lifetime).”
- Sahih Muslim, The Virtues of the Companions, Number 5975
- The Quran 96:01-05 (Surah al-Alaq)
- Sahih Muslim, The Virtues of the Companions, Number 5970
- Sahih Bukhari, Volume 5, Book 58, Number 165
- Sahih Bukhari, Volume 5, Book 58, Number 163