Sayyiduna al-Bara' (may Allah be pleased with him) said: The Ansar used to give some poor-quality dates in zakah as well, so this verse was revealed: "O you who believe, spend from the good things you have earned and from what We have brought forth for you from the earth, and do not aim for the bad (of it), spending from it, while you yourselves would not take it except with closed eyes." [Surah al-Baqarah: 267]. The narrator said: Dun means something worthless and inferior. If you have a right due from someone and he gives you something less, he has reduced your right; if you accept it, that is ighmad (closing the eyes).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7528