مَّن يُطِعِ ٱلرَّسُولَ فَقَدْ أَطَاعَ ٱللَّهَ ۖ وَمَن تَوَلَّىٰ فَمَآ أَرْسَلْنَٰكَ عَلَيْهِمْ حَفِيظًۭا ﴿٨٠﴾
He who obeys the Messenger has obeyed Allah; but those who turn away - We have not sent you over them as a guardian.
Short answer
The Sunnah is the way of Prophet Muhammad ﷺ: what he said, did and approved. The hadith are the reports that carry it, and they are the second source of Islamic teaching after the Quran.
The fuller answer
The Quran gives principles, and the Sunnah explains how to apply them. For example, the Quran commands prayer, and the hadith show how to pray, how many units and at what times. The Quran tells Muslims to obey the Messenger.
Hadith were collected carefully. Each report has a chain of narrators, and scholars checked each narrator's honesty and memory. The most trusted collections are Sahih al-Bukhari and Sahih Muslim, followed by the Sunan of Abu Dawud, at-Tirmidhi, an-Nasa'i and Ibn Majah. This website's hadith library shows the grade of each hadith, authentic (sahih), good (hasan) or weak (da'if), and its source.
From the Quran
مَّآ أَفَآءَ ٱللَّهُ عَلَىٰ رَسُولِهِۦ مِنْ أَهْلِ ٱلْقُرَىٰ فَلِلَّهِ وَلِلرَّسُولِ وَلِذِى ٱلْقُرْبَىٰ وَٱلْيَتَٰمَىٰ وَٱلْمَسَٰكِينِ وَٱبْنِ ٱلسَّبِيلِ كَىْ لَا يَكُونَ دُولَةًۢ بَيْنَ ٱلْأَغْنِيَآءِ مِنكُمْ ۚ وَمَآ ءَاتَىٰكُمُ ٱلرَّسُولُ فَخُذُوهُ وَمَا نَهَىٰكُمْ عَنْهُ فَٱنتَهُوا۟ ۚ وَٱتَّقُوا۟ ٱللَّهَ ۖ إِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ ﴿٧﴾
And what Allah restored to His Messenger from the people of the towns - it is for Allah and for the Messenger and for [his] near relatives and orphans and the [stranded] traveler - so that it will not be a perpetual distribution among the rich from among you. And whatever the Messenger has given you - take; and what he has forbidden you - refrain from. And fear Allah; indeed, Allah is severe in penalty.
Please note
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.