وَعَنْ جَابِرٍ
قَالَ: أَرَدْتُ الْخُرُوجَ إِلَى خَيْبَرَ فَأَتَيْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَسَلَّمْتُ عَلَيْهِ وَقُلْتُ: إِنِّي أَرَدْتُ الْخُرُوجَ إِلَى خَيْبَرَ فَقَالَ: «إِذَا أَتَيْتَ وَكِيلِي فَخُذْ مِنْهُ خَمْسَةَ عَشَرَ وَسْقًا فَإِنِ ابْتَغَى مِنْكَ آيَةً فَضَعْ يدك على ترقوته»
Jabir said he intended to go to Khaibar, so he went to the Prophet (sallallahu alayhi wa sallam), gave him a salutation and told him of his purpose. He replied, “When you come to my agent take fifteen camel-loads from him, and if he desires a sign from you put your hand on his collar-bone*”. *This was evidently a pre-arranged sign to show that the Messenger was genuine.
Mishkat al-Masabih 2815, Book 12, Business Transactions, Hadith 171. 1 min read