Sahih Bukhari
Oaths and Vows
Volume: 008 / Book Number: 078 / Hadith Number: 681
Narrated Ka'b bin Malik:
In the last part of his narration about the three who remained behind (from the battle of Tabuk). (I said) "As a proof of my true repentance (for not joining the Holy battle of Tabuk), I shall give up all my property for the sake of Allah and His Apostle (as an expiation for that sin)." The Prophet said (to me), "Keep some of your wealth, for that is better for you."