Abdullah bin Mas'ud (may Allah be pleased with him) was the sixth to accept Islam

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: "I saw myself in a state where I was the sixth person to accept Islam, and at that time there was no other Muslim on the face of the earth besides us."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Urwah bin al-Zubayr (may Allah be pleased with him) that among the allies of Banu Zuhrah bin Kilab who took part in the Battle of Badr was Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him). Urwah further narrates that among those who made the first emigration to Abyssinia, before the departure of Sayyiduna Ja'far bin Abi Talib (may Allah be pleased with him), was Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him).

Abdullah bin Sakhbarah narrates: I was with Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him). He was of a brown complexion, there was a special dignity and charm about his person, and he was a man of slender and lean build.

Khalifah bin Khayyat narrates that Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) passed away in Madinah, and Sayyiduna al-Zubayr bin al-Awwam (may Allah be pleased with him) led his funeral prayer.

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) established brotherhood (mu'akhah) between Sayyiduna al-Zubayr bin al-Awwam and Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with them both).
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.