Abu Amr al-Awza'i (may Allah have mercy on him) says that Ibn Shihab al-Zuhri (may Allah have mercy on him) said: The Sunnah has been established regarding the zakah of olives that it is taken from whoever presses his olives, at the time he presses them: on what is watered by the sky and the rivers, or is rain-fed (ba'l), a tithe; and on what is watered by drawing water with a water-camel's rope, half a tithe.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7456