Regarding authorization
باب فِي الْوَكَالَةِ
Sunan Abu Dawood, Book of The Office of the Judge (Kitab Al-Aqdiyah)(view book)
Sunan Abu Dawood 3632
Narrated Jabir ibn Abdullah:
I intended to go (on expedition) to Khaybar. So I came to the Holy Prophet ﷺ, greeted him and said: I am intending to go to Khaybar. He said: When you come to my agent, you should take from him fifteen wasqs (of dates). If he asks you for a sign, then place your hand on his collar-bone.
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ سَعْدِ بْنِ إِبْرَاهِيمَ، حَدَّثَنَا عَمِّي، حَدَّثَنَا أَبِي، عَنِ ابْنِ إِسْحَاقَ، عَنْ أَبِي نُعَيْمٍ، وَهْبِ بْنِ كَيْسَانَ عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ، أَنَّهُ سَمِعَهُ يُحَدِّثُ، قَالَ أَرَدْتُ الْخُرُوجَ إِلَى خَيْبَرَ فَأَتَيْتُ رَسُولَ اللَّهِ ﷺ فَسَلَّمْتُ عَلَيْهِ وَقُلْتُ لَهُ إِنِّي أَرَدْتُ الْخُرُوجَ إِلَى خَيْبَرَ . فَقَالَ " إِذَا أَتَيْتَ وَكِيلِي فَخُذْ مِنْهُ خَمْسَةَ عَشَرَ وَسْقًا فَإِنِ ابْتَغَى مِنْكَ آيَةً فَضَعْ يَدَكَ عَلَى تَرْقُوَتِهِ " .