Muhammad bin Rashid (may Allah have mercy on him) narrates from Makhul (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not buy the zakah charities until they have been marked and taken into possession."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7630