Malik narrates from Ibn Shihab that the first person to take zakah from stipends was Amir Mu'awiyah (may Allah be pleased with him). Imam al-Shafi'i (may Allah have mercy on him) said: It is an acquired gain (fa'idah); there is no zakah on it until a year has passed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7357