Abdullah bin Masud (RA) once said: 
The first people to declare their Islam publicly were seven: The Messenger of Allah (PBUH), Abu Bakr, Ammar and his mother Sumayyah, Suhaib, Bilal and Miqdad.
Hz Sumayyah bint Khayyat (RA) was the first martyr woman companion in Islam. She was under the service of Abu Hudhayfah ibn al-Mughirah, who later gave her in marriage to Yasir bin Ammar (RA), a Yemeni who came to Mecca and started living there. She was blessed with a son, who was named after Yasir’s father, Ammar bin Yasir.
Hz Sumayyah (RA), along with her husband and son, were the earliest people who believed in the prophethood and message of Prophet Muhammad (PBUH). She was the seventh person to embrace Islam. Hz Sumayyah (RA) and members of her family did not keep their belief a secret. They declared themselves to be Muslims in public. When the unbelievers got to know that they had embraced Islam, Hz Sumayyah (RA) and her family were taken to the burning desert of Mecca. They were tortured, both physically and mentally, by the Quraysh who were trying to convert them back to paganism. However, Hz Sumayyah (RA) maintained a firm stance.
Prophet Muhammad (PBUH) used to go where they were tortured, commending their courage by saying “Keep patient, Yasir’s family. Verily, your meeting place will be in Paradise”.
They suffered extreme hardship which tested their faith, but they persevered and remained patient, knowing that the path of truth is often filled with difficulties.
Eventually, being old, Hz Yasir (RA) could not endure the torture, and died in the way of Allah and His messenger (PBUH). Although her husband, Hz Yasir (RA) was martyred before her eyes, Quraysh pagans failed to convert Hz Sumayyah (RA). After torturing her for a long period of time, Abu Jahl stabbed her with his spear, causing her to attain martyrdom.
While Yasir (RA) and his wife Sumayyah (RA) were killed when they refused to denounce Islam, their son Ammar bin Yasir was spared when he accepted to denounce Islam. Regarding this incident, Allah revealed the below-mentioned verse: 
Whosoever disbelieves in Allah after his belief — except him who is forced thereto and whose heart is still content with the Faith — but whosoever finds ease in disbelief: On them is Wrath from Allah. Theirs will be an awful doom.
Hz Sumayyah (RA) and her husband became the first martyrs in Islam. For such people, Allah revealed in The Quran: 
Lo! Allah has bought from the believers their lives and their wealth because the Garden will be theirs: they shall fight in the way of Allah and shall slay and be slain. It is a promise which is binding on Him in the Torah and the Gospel and the Quran. Who fulfills the covenant better than Allah? Rejoice then in your bargain that ye have made, for that is the supreme triumph.
- Ibn Majah Volume 1, Book 1, Hadith 15
- The Quran 16:106 (Surah an-Nahl)
- The Quran 09:111 (Surah at-Tawbah)