Amr bin Shu'ayb narrates from his father, from his grandfather, that the Prophet (peace and blessings be upon him) said: "There is no zakah on working camels."
This is how Ghalib al-Qattan narrated it. The same has been narrated concerning cattle from Ibn Abbas (may Allah be pleased with them both) as marfu' (from the Prophet) and from Mu'adh bin Jabal (may Allah be pleased with him) as mawquf (his own statement), and both chains contain weakness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7391