Suwayd bin Ghafalah (may Allah be pleased with him) said: The collector of the Prophet (peace and blessings be upon him) came, so I took his hand and he took my hand, and I read in his document: "Those grazing together are not to be separated, and those grazing separately are not to be combined, for fear of zakah." A man came with a large she-camel, and he refused to take it; then he brought another, and he refused that too; then he brought yet another, and he refused that too, and said: "Which earth will carry me and which sky will shade me when I come to the Messenger of Allah (peace and blessings be upon him) having taken the best camels of a Muslim man?" It has already passed in the hadith of Ubayy bin Ka'b (may Allah be pleased with him), when he went out as a collector, and in it is evidence that it is permissible to take what is given voluntarily.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7307