What you say to someone that rejects the ahadeeth, is, that "Actually, in doing so, you are rejecting the Qur'an as well."
This can be shown using aayaat from the Quran (as they claim to believe in or obey only the Qur'an):
"Say: If you really love Allah, follow me. Allah will love you and forgive you your sins. Allah is forgiving, Merciful" (3:31)
"Say:
Obey Allah and obey the messenger.
But if they turn away, Allah loves not the disbelievers." (3:32)
"
Obey Allah, and obey the Messenger, and beware (of evil): if ye do turn back, know ye that it is Our Messenger’s duty to proclaim (the Message) in the clearest manner". (5: 92).
"O ye who believe!
obey Allah, and obey the Messenger, and those charged with authority among you. If ye differ in anything among yourselves, refer it to Allah and His Messenger, if ye do believe in Allah and the Last Day: that is best, and most suitable for final determination." (4: 59).
"Say: “
Obey Allah, and obey the Messenger: but if ye turn away, he is only responsible for the duty placed on him and ye for that placed on you.
If ye obey him, ye shall be on right guidance".(24: 54).
"And
whoever obeys Allâh and His Messenger, Allâh shall admit him in the Gardens underneath which rivers flow." (4:13)
"And
whoever obeys Allâh and His Messenger, he has won a great success." (33:71)
"And
we sent no messenger, but that he should be obeyed by the leave of Allâh." (4:64)
Allah is telling us in the Qur'an to obey the messenger (sallallahu alayhi wa sallam) . So how can one obey the messenger? The ONLY way of doing that is to follow his doings and sayings, which are recorded in the ahadeeth.
Now, they might say:
“that is not evidance for following his sunnah, obeying the prophet (sallallahu alayhi wa sallam) means obeying what is in Quran that he came with from Allah”
That is incorrect, because the Quran is Allah’s words not the Prophets (sallallahu alayhi wa sallam) words, so obeying what is in the Quran is obeying Allah ‘azza wa jal.
It can’t mean obeying the Prophet (sallallahu alayhi wa sallam) because it is not his words, so it is not his commands.
Also, where in the Qur'an does it tell you the specifics of how to pray, how many raka'ahs what to say in rukoo', sujood? Nowhere. That's in the ahadeeth. So are the ahadeeth rejectors going to make up their own method to pray?
Also, look at this:
And whoever obeys the Messenger, thereby obeys Allâh. (4:80)
1.Allah has commanded us in Quran to obey the Messenger (sallallahu alayhi wa sallam), so obeying the messenger (sallallahu alayhi wa sallam) would be obeying Allah’s command.
2.The Prophet’s (sallallahu alayhi wa sallam) teachings are not from himself, but from Allah, He is the one who taught the Prophet (sallallahu alayhi wa sallam) to teach the believers, so obeying the Prophet’s teachings would be obeying Allah.
The only way you can obey the messenger (sallallahu alayhi wa sallam) is by obeying what he said and did, which is recorded in the ahadeeth. If they do not believe in the ahadeeth and reject it, then they are rejecting Allah's command in the Qur'aan (shown in mumerous verses above) and thus, have, in reality, rejected the Qur'aan too.
Allah knows best.
