Musannaf Ibn Abi Shayba 28:207

Muhammad ibn Fudayl narrated to us, from his father, from Ibn Abi Mulaykah, who said: Umar said, "By the One in whose hand is my soul, if Abu Bakr had obeyed us, we would have disbelieved in a single morning when they asked for a reduction in the Zakat, but he refused them." He said, "If they had withheld from me a hobbling rope, I would have fought them."