It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The first matter to be judged between people will be (cases of) bloodshed, and the first thing for which a servant will be held to account will be the prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10387