ENOfficial
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.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مَسْرُوقُ بْنُ الْمَرْزُبَانِ، حَدَّثَنَا يَحْيَى بْنُ أَبِي زَائِدَةَ، عَنْ هَاشِمِ بْنِ هَاشِمٍ، قَالَ سَمِعْتُ سَعِيدَ بْنَ الْمُسَيَّبِ، يَقُولُ قَالَ سَعْدُ بْنُ أَبِي وَقَّاصٍ مَا أَسْلَمَ أَحَدٌ فِي الْيَوْمِ الَّذِي أَسْلَمْتُ فِيهِ وَلَقَدْ مَكَثْتُ سَبْعَةَ أَيَّامٍ وَإِنِّي لَثُلُثُ الإِسْلاَمِ .
sunnah.com