Abu Dawud (may Allah have mercy on him) narrated this hadith from Sa'id bin Mansur (may Allah have mercy on him) and mentioned the whole hadith.
It is narrated that the Messenger of Allah (peace and blessings be upon him) said to Umar (may Allah be pleased with him): "We have already taken one year's zakah from Abbas (may Allah be pleased with him) in advance." Also, in the story of Umar and Abbas (may Allah be pleased with them both) it is reported: "We hastened in taking the zakah of Abbas's (may Allah be pleased with him) wealth; in this year we took the coming year's zakah."
This (last, mursal) narration is the most authentic of these narrations.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7366