Hadith #3632 (Sunan Abi Dawud)
Book: The Office of the Judge (Kitab Al-Aqdiyah)
Narrator(s): Abu Hurairah, Bashyr bin Ka'b bin Aby بشير بن كعب بن أبي, Muslim bin Ibrahim al-Azdy مسلم بن إبراهيم الأزدي الفراهيدي, Qatada قتادة
Grade: Da'if

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.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-3632/