Aqil said that al-Zuhri (may Allah have mercy on him) narrated this hadith and said that Abu Bakr (may Allah be pleased with him) said: By Allah! If they withhold even one young goat (and in one narration: even one rope), I will fight.
Abu Dawud (may Allah have mercy on him) said that Abu Ubaydah Ma'mar bin al-Muthanna said: 'Iqal is a year's zakah, and 'iqalan is two years' zakah. The Shaykh said: Taking a young goat is not conceivable except in the case we have described.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7325