Sahih al-Bukhari 1502

Narrated Anas bin Malik:I took `Abdullah bin Abu Talha to Allah's Messenger (ﷺ) 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 (ﷺ) who would chew a piece of date and put a part of its juice in the child's mouth). I saw the Prophet (ﷺ) and he had an instrument for branding in his hands and was branding the camels of Zakat.