Abu al-Zubayr (may Allah have mercy on him) said that Jabir (may Allah be pleased with them both) said: Zakah is not to be collected from the cows with which ploughing is done. This chain is sahih (authentic), and this is the view of Mujahid, Sa'id bin Jubayr, Umar bin Abd al-Aziz and Ibrahim al-Nakha'i. Al-Hasan al-Basri (may Allah have mercy on him) said: There is no zakah on working oxen when they are in a city.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7397