Sahih Bukhari, Chapter: Obligatory Charity Tax (Zakat)

I took Abdullah bin Abu Talha to Allah's Apostle to perform Tahnik for him. (Tahnik was a custom among the Muslims that whenever a child was born they used to take it to the Prophet who would chew a piece of date and put a part of its juice in the child's mouth). I saw the Prophet and he had an instrument for branding in his hands and was branding the camels of Zakat.