Jami` at-Tirmidhi, Book 23, Hadith 45 (In-Book) ⓘ

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 ‏.‏