narrated talha bin 'ubaid-ullah (radi-allahu 'anhu):

a bedouin with unkempt hair came to allah's apostle (sallallahu 'alaihi wa sallam) and said, "o allah's apostle! inform me what allah has made
compulsory for me as regards the prayers." he replied: "you have to offer perfectly the five compulsory prayers in a day and night (24 hours), unless you want to pray nawafil." the bedouin further asked, "inform me what allah has made compulsory for me as regards fasting." he replied, "you have to fast during the whole month of ramadan, unless you want to fast more as
nawafil." the bedouin further asked, "tell me how much zakat allah has enjoined on me." thus, allah's apostle (sallallahu 'alaihi wa sallam)
informed him about all the rules (i.e. fundamentals) of islam. the bedouin then said, "by him who has honored you, i will neither perform any nawafil
nor will i decrease what allah has enjoined on me. allah's apostle (sallallahu 'alaihi wa sallam) said, "if he is saying the truth, he will succeed (or he will be granted paradise)."

bukhari vol. 3 : no. 115