Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) passed away and Sayyiduna Abu Bakr (may Allah be pleased with him) took on the caliphate after him (peace and blessings be upon him), and those among the Arabs who were to disbelieve (in the religion) did so, Sayyiduna Umar (may Allah be pleased with him) said: O Abu Bakr (may Allah be pleased with him)! How will you fight the people when the Messenger of Allah (peace and blessings be upon him) said: "I have been commanded to fight the people until they affirm 'La ilaha illallah' (There is no god worthy of worship except Allah); and whoever affirms 'La ilaha illallah' (There is no god worthy of worship except Allah) has protected his wealth and his life from me, except by its right, and then his reckoning is with Allah Almighty." Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) said: "I will fight even the one who separates between prayer and zakah, for zakah is the right of wealth. By Allah! If they withhold from me even one young kid of zakah that they used to pay in the time of the Messenger of Allah (peace and blessings be upon him), I will fight them for withholding it." Sayyiduna Umar (may Allah be pleased with him) said: By Allah! I saw that Allah Almighty had opened his heart to the truth for fighting, until I knew that this was the truth (correct).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7324
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