Abu Dawud Book 18, Hadith Number 3625.

Narated By Jabir ibn Abdullah : I intended to go (on expedition) to Khaybar. So I came to the Holy Prophet (PBUH), 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.