It was narrated that Hashim bin Hashim said:
"I heard Sa'eed bin Musayyab say: 'Sa'd bin Abu Waqqas said: 'No one else became Muslim on the same day as I did; for seven days I was one-third of Islam.'" (ID)
English reference (ID): Vol. 1, Book 1, Hadith 132 (ID)
Arabic reference (ID): Book 1, Hadith 137 (ID)