Musnad Ahmad 175:7

Rawh narrated to us, Ibn Jurayj narrated to us, he said: Ibn Shihab informed me, from Abd al-Rahman ibn Abd Allah ibn Ka'b ibn Malik, that when Allah accepted the repentance of Ka'b ibn Malik, he came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "Indeed, Allah has not saved me except through truthfulness, and it is part of my repentance to Allah that I shall never tell a lie again. And I strip myself of my wealth as charity for the sake of Allah and His Messenger." The Messenger of Allah, peace and blessings of Allah be upon him, said to him: "Keep some of your wealth, for it is better for you." He said: "Then I shall keep my share from Khaybar."