Sahih Bukhari, Chapter: Sales and Trade

The Prophet said, "Don't keep camels and sheep unmilked for a long time, for whoever buys such an animal has the option to milk it and then either to keep it or return it to the owner along with one Sa of dates." Some narrated from Ibn Sirin (that the Prophet had said), "One Sa of wheat, and he has the option for three days." And some narrated from Ibn Sirin, " ... a Sa of dates," not mentioning the option for three days. But a Sa of dates is mentioned in most narrations.