a) (V.9:5) See the footnote of (V.2:193).
b) Narrated Abû Hurairah رضي الله عنه : When the Prophet صلى الله عليه وسلم died and Abû Bakr became his successor and some of the Arabs reverted to disbelief, ‘Umar said, "O Abû Bakr! How can you fight these people although Allâh’s Messenger صلى الله عليه وسلم said, " I have been ordered to fight the people till they say: La ilâha illallâh (none has the right to be worshipped but Allâh), and whoever said La ilâha illallâh will save his property and his life from me, unless (he does something for which he receives legal punishment) justly, and his account will be on Allâh?" Abû Bakr said, "By Allâh! I will fight who ever differentiates between Salât (prayers) and Zakât, as Zakât is the right to be taken from property (according to Allâh’s orders). By Allâh! If they refused to pay me even a kid they used to pay to Allâh’s Messenger صلى الله عليه وسلم, I would fight with them for withholding it," ‘Umar said "By Allâh! It was nothing, but I noticed that Allâh opened Abû Bakr’s chest towards the decision to fight, therefore I realized that his decision was right." (Sahih Al-Bukhâri, Vol.9, Hadîth No 59).