Ibn Shihab (may Allah have mercy on him) narrates that Abu Bakr and Umar (may Allah be pleased with them both) did not take zakah doubled (two years at once); rather, they sent collectors for it in drought and in plenty, in times of fatness and of leanness, because the beloved Prophet (peace and blessings be upon him) collected zakah every year. Therefore this is the Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7359