Sahih Bukhari Volume 2, Book 24, Hadith 1401

فَقَالَ وَاللَّهِ لأُقَاتِلَنَّ مَنْ فَرَّقَ بَيْنَ الصَّلاَةِ وَالزَّكَاةِ، فَإِنَّ الزَّكَاةَ حَقُّ الْمَالِ، وَاللَّهِ لَوْ مَنَعُونِي عَنَاقًا كَانُوا يُؤَدُّونَهَا إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم لَقَاتَلْتُهُمْ عَلَى مَنْعِهَا‏.‏ قَالَ عُمَرُ ـ رضى الله عنه ـ فَوَاللَّهِ مَا هُوَ إِلاَّ أَنْ قَدْ شَرَحَ اللَّهُ صَدْرَ أَبِي بَكْرٍ ـ رضى الله عنه ـ فَعَرَفْتُ أَنَّهُ الْحَقُّ‏

Abu Bakr said, "By Allah! I will fight those who differentiate between the prayer and the Zakat as Zakat is the compulsory right to be taken from the property (according to Allah's orders) By Allah! If they refuse to pay me even a she-kid which they used to pay at the time of Allah's Apostle. I would fight with them for withholding it" Then 'Umar said, "By Allah, it was nothing, but Allah opened Abu Bakr's chest towards the decision (to fight) and I came to know that his decision was right."