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Ibn Hijr (may Allaah have mercy on him) says in his book Al-Isaabah fi Tamyeez al-Sahaabah, where he gives a biography of Sumayah:

The daughter of Khabbaat and the mother of ‘Ammaar ibn Yaasir, she was the seventh person to enter Islam. She was tortured by Abu Jahl who stabbed her in her private parts, and she died. She was the first shaheedah (martyr) in Islaam…

Al-Mujaahid said: "The first seven to publicly declare their Islam in Makkah were: the Messenger of Allaah (peace and blessings of Allaah be upon him), Abu Bakr, Bilaal, Khabbaab, Suhayb, ‘Ammaar and Sumaya. The Messenger of Allaah (peace and blessings of Allaah be upon him) and Abu Bakr were protected by their own people, but the others were forced to wear iron shields then were exposed to the burning sun. Abu Jahl came to Sumaya and stabbed her in her private parts, killing her." This was reported by Abu Bakr ibn Abi Shaybah from Mujaahid; it is mursal and its isnaad is saheeh.

Ibn Sa’d reported with a saheeh isnaad from Mujaahid: "The first martyr in Islam was Sumayah, the mother of ‘Ammaar ibn Yaasir. She was an old, weak woman (according to a report narrated by al-Bayhaqi: Abu Jahl stabbed her in her private parts. Al-Dalaa’il, 2/282). When Abu Jahl was killed on the day of Badr, the Prophet (peace and blessings of Allaah be upon him) said (to ‘Ammaar): ‘Allaah has killed the one who killed your mother.’" (al-Isaabah, 4/327; al-Bidaayah wa’l-Nihaayah by Ibn Katheer, 3/59).

Taken from Islam Questions & Answers