Jabir bin Abdullah (may Allah be pleased with them both) said: I intended to go to Khaybar, so I came to the Prophet (peace and blessings be upon him), greeted him with salam and said that I intended to go to Khaybar. He (peace and blessings be upon him) said: "When you go to my agent, take fifteen wasqs of dates from him. If he asks you for a sign, place your hand on his throat."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوكالة / 11432