It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) walked with them up to Baqi' al-Gharqad, then bade them farewell and said: "Go in the name of Allah. O Allah! Help these people." By these people, he (peace and blessings be upon him) meant those whom he had sent to kill Ka'b bin Ashraf.
Hadith Referenceالفتح الربانی / كتاب الجهاد / 4954