Dear noble brothers, we are discussing a new lesson of this series about the Ayaat that start with, "O you who believe…", and the Ayah today is the following:  

(O you who believe! Obey Allah and obey the Messenger (Muhammad PBUH), and those of you (Muslims) who are in authority. (And) if you differ in anything amongst yourselves, refer it to Allah and His Messenger (PBUH), if you believe in Allah and in the Last Day. That is better and more suitable for final determination.)

[An-Nisa', 59]

Man sometimes is under someone who is more powerful than him, and thus he may get confused, and he is unable to decide whom he should obey and whom he should disobey, and so this Ayah refers clearly and decisively to this serious issue in religion, indicating the party we should obey and the one we should not. Allah the Almighty says:  

(Obey Allah and obey the Messenger (Muhammad PBUH)…)

[An-Nisa', 59]

The Scholars of Ususl Ad-Din (fundamentals of Islam) say that when the word "Messenger, peace be up him", comes after the word "obey", it means we should obey the Noble Prophet independently, because he is infallible. Due to this fact, Allah, Exalted and Sublime be He, says:    

(And whatsoever the Messenger (Muhammad PBUH) gives you, take it, and whatsoever he forbids you, abstain (from it), and fear Allah.)

[Al-Hashr, 7]

Accordingly, we should obey Allah's Divine Commands in the noble Quran, and we should obey the authenticated Sunnah of the Messenger, peace be upon him: Sunnah Qawliyyah (the verbal commands of the Noble Prophet), Sunnah Fi'liyyah (his actions), and Sunnah Iqrariyyah (the matters he approved).