And Allah ta`ala said:

“Believers! Obey Allah, and obey the Messenger and those charged with authority among you; if you differ in anything, refer it to Allah and the Messenger (for judgment) if you truly believe in Allah and the Last Day: that is best and most suitable for final determination.” [4:59]

Allah ta`ala is requiring obedience to Him and to His Messenger (sallallahu `alayhi wa sallam). He starts the ayah with an address to the “believers”, hinting that what is required thereafter is a consequence of that name with which they are addressed.

This is similar to saying: “You whom Allah has favored and enriched with His bounties, be good to others as Allah has been good to you.” And like, “Learned men, teach people what would benefit them.” And, “Ruler, rule with justice.” And so o­n.

For this reason, legislative matters in the Qur’an are frequently addressed to the believers, starting the address with “Believers”. For example Allah says:

“Believers, fasting is prescribed for you.” [2:183]

And He says:

“Believers, when the call is proclaimed for prayer o­n Friday, hasten earnestly to the remembrance of Allah.” [62:9]

And He says:

“Believers, fulfill the contracts.” [5:1]

Addressing the believers like this carries the implication that: “If you are true believers, you should perform the following action, because it is a requirement for the integrity and sincerity of Iman.”