Sa'd (may Allah be pleased with him) was counted as a third of Islam

Amir bin Sa'd narrates from his father that he said: "I saw myself in a state where I was the third of those who had accepted Islam (that is, one third of all the Muslims)."

It is narrated from Sayyiduna Sa'd bin Abi Waqqas (may Allah be pleased with him) that he said: "On the day I accepted Islam, no one other than me accepted Islam, and I remained for seven nights in the state of being one third of Islam."
The chain of this hadith is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.

Aishah bint Sa'd narrates from her father that the Prophet (peace and blessings be upon him) sat in the mosque for three nights, saying: 'Allahumma adkhil min hadha al-babi 'abdan yuhibbuka wa tuhibbuhu' (O Allah, let a servant enter through this door who loves You and whom You love). Then Sayyiduna Sa'd (may Allah be pleased with him) entered through it.
The chain of this hadith is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it.