some people argue that ahadith only compiled on the 3rd century so we cant rely upon the authenticity.how can we explain them?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Recording the Prophetic narrations passed through different stages, the most important of which are:

The First Stage: it started during the era of the Prophet as confirmed in many authentic narrations. It is confirmed that ‘Abdullaah ibn ‘Amr ibn Al-‘Aas used to write the narrations of the Prophet . In a narration by ‘Ali –as reported by Al-Bukhari and Muslim –he said ‘what is in this piece of paper’ and then he read out some Prophetic narrations which were written on that paper. Also, the Prophet said: “Write for Abu Shaah", and other evidence which proves that the companions used to record the narrations of the Prophet .

The Second Stage: it started in the second Hijri year during the reign of ‘Umar ibn ‘Abdul-‘Azeez when the Muslims started to record all the Prophetic narrations during his caliphate.

All the narrations reported that he was the first to compile the Prophetic narrations, as he sent to the governor and judge of Al-Madeenah, Abu Bakr ibn Hazm telling him: “Write all the narrations of the Prophet because I fear knowledge will disappear by the death of the scholars.” He also ordered him to write the narrations from ‘Amrah bint ‘Abdur-Rahmaan and Al-Qaasim ibn Muhammad .

In addition to this, he asked Imaam Muhammad Ibn Muslim Az-Zuhri to compile the remaining of the Prophetic narrations which the people of Madeenah had. Moreover, he sent to the rulers of all the countries and to their great scholars asking them to do the same thing.

Therefore, this is not defamation for the authenticity of the Prophetic narrations because even though they were not entirely written [at the time of the Prophet, sallallaahu ‘alayhi wa sallam], people memorized them and learnt them by heart and they used to study them by repeating them orally.

The companions received the narrations from the Prophet and they transmitted them to the generation which came after them, and then they started writing them down gradually and the generation which came after the companions transmitted them to the coming generations until they reached us today with authentic chain of narrators; so there is no way to disparage the Prophetic narrations.

For more benefit, please refer to Fataawa 86092 and 85821.

Allaah Knows best.