Skynightblaze and Darth say that 33:21 is a verse which tells all people to follow Muhammad's personal example (recorded in ahadith).
Here it is in context:
"Allah knows indeed those among you who hinder others and those who say to their brethren: Come to us; and they come not to the fight but a little, being niggardly with respect to you; but when fear comes, you will see them looking to you, their eyes rolling like one swooning because of death; but when the fear is gone they smite you with sharp tongues, being niggardly of the good things. These have not believed, therefore Allah has made their doing naught; and this is easy to Allah. They think the allies are not gone, and if the allies should come [again] they would fain be in the deserts with the desert Arabs asking for news about you, and if they were among you they would not fight save a little. Certainly you have in the Messenger of Allah an excellent exemplar for him who hopes in Allah and the latter day and remembers Allah much. And when the believers saw the allies, they said: This is what Allah and His Messenger promised us, and Allah and His Messenger spoke the truth; and it only increased them in faith and submission. Of the believers are men who are true to the covenant which they made with Allah: so of them is he who accomplished his vow, and of them is he who yet waits, and they have not changed in the least that Allah may reward the truthful for their truth, and punish the hypocrites if He please or turn to them [mercifully]; surely Allah is Forgiving, Merciful."
Qur'an 33:18-24
So here is my response to them:
33:18-20 describes the poor behaviour of hypocrites in a situation of conflict.
33:21 says a good example was set by the Messenger for those who hope in Allah and remember Him much.
33:22 describes the behaviour of true believers in the same situation.
33:23 contrasts Muslim behaviour to that of the hypocrites (true Muslims are true to the covenant, whereas hypocrites flee).
33:24 inform us that a good example is in not doing what the hypocrites did.
Therefore 33:21 refers to the good example of the Messenger in displaying courage and trust in Allah.
Demonstrating a good example for those who do a particular thing means setting a good example in doing that thing.
Precisely the same expression is used for Prophet Ibrahim (60:6) in another contextual situation (60:4-6).
60:6 does not mean we need ahadith of Prophet Ibrahim, thus 33:21 does not mean we need ahadith of Prophet Muhammad.
The “good example” comes from conduct conforming to the ideals detailed by the Qur’an.
For instance: Prophet Ibrahim’s behaviour (9:114, 11:69, 11:75, 19:42-48).
Muhammad only followed what was revealed to him (2:170, 6:50, 6:106, 7:3, 7:203, 10:15, 10:109, 33:2, 46:9), and thus the example is an example in following the Qur'an. The example is described as one in "believing in the Last Day and remembering Allah much", but if one studies the Qur'an, they will see that it tells people to believe in the Last Day, and how to remember Allah. Someone who follows the Qur'an will be a good example in doing what the Qur'an teaches.
Yet people still use 33:21 to justify ahadith (because it pleases them).
Which hadith would they believe in after this?
Qur'an 45:6