Sahih Muslim 1856
Sahih Muslim · Book 33: The Book on Government · It is recommended for the army to swear allegiance to the ruler when intending to fight, and an account of Bay'at Ar-Ridwan beneath the tree
It has been narrated (through a different chain of transmitters) on the authority of Salim b. al-Ja'd who said:
وَحَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ ، قَالَ إِسْحَاقُ أَخْبَرَنَا وَقَالَ، عُثْمَانُ حَدَّثَنَا جَرِيرٌ ، عَنِ الأَعْمَشِ ، حَدَّثَنِي سَالِمُ بْنُ أَبِي الْجَعْدِ ، قَالَ قُلْتُ لِجَابِرٍ كَمْ كُنْتُمْ يَوْمَئِذٍ قَالَ أَلْفًا وَأَرْبَعَمِائَةٍ .
I asked Jabir: How many were you on the Day of Hudaibiya? He said: One thousand and four hundred.