كتاب الجهاد عن رسول الله صلى الله عليه وسلم
23  
The Book on Jihad
(31)
Chapter: What Has Been Related About the (Age) Of Adulthood For A Man And When He Is To Recieve A Salary
(31)
باب مَا جَاءَ فِي حَدِّ بُلُوغِ الرَّجُلِ وَمَتَى يُفْرَضُ لَهُ‏)‏
Jami` at-Tirmidhi 1711

Another chain with similar, but he (Nafi') said:

"Umar [bin 'Abdul-'Aziz] said: 'This is the limit that distinguished between children and soldiers.'" And he did not mention him writing about the salary

[Abu 'Eisa said:] The Hadith of Ishaq bin Yusuf is a Hasan Sahih Gharib Hadith as narration of Sufyan Ath-Thawri

حَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عُبَيْدِ اللَّهِ، نَحْوَهُ بِمَعْنَاهُ إِلاَّ أَنَّهُ قَالَ قَالَ عُمَرُ بْنُ عَبْدِ الْعَزِيزِ هَذَا حَدُّ مَا بَيْنَ الذُّرِّيَّةِ وَالْمُقَاتِلَةِ ‏.‏ وَلَمْ يَذْكُرْ أَنَّهُ كَتَبَ أَنْ يُفْرَضَ ‏.‏ قَالَ أَبُو عِيسَى حَدِيثُ إِسْحَاقَ بْنِ يُوسُفَ حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ مِنْ حَدِيثِ سُفْيَانَ الثَّوْرِيِّ ‏.‏
Grade: Sahih (Darussalam)
Reference : Jami` at-Tirmidhi 1711
In-book reference : Book 23, Hadith 45
English translation : Vol. 3, Book 21, Hadith 1711