Ibn Majah

Commentary:
(1)
The Prophetic hadiths are a legal proof in Shariah, therefore the noble Companions (radi Allahu anhum) would listen to and memorize the hadiths with utmost effort and diligence.
Abdullah ibn Abbas (radi Allahu anhu) was passing through childhood during the lifetime of the Prophet (sallallahu alayhi wa sallam), so he was able to hear very few hadiths directly from him. However, later on, he listened to and memorized many hadiths from the senior Companions (radi Allahu anhum), to the extent that he came to be counted among the Companions who narrated many hadiths.

(2)
The original proof is only the statement of the Prophet; the fatwas of other individuals cannot have the same status. Therefore, it is necessary to seek evidence from the Qur'an and hadith for every issue.

(3)
The statements of the Companions and Followers (Tabi'in) can be used where a Prophetic hadith is not found.
For this reason, the Followers would also write down the statements of the Companions (radi Allahu anhum).
Ibn Abbas (radi Allahu anhu) did not consider it appropriate that the statements of others be written alongside the Prophetic hadith, and thus he drew attention to the fact that hadiths should be memorized, not statements and fatwas.

(4)
Walking on hard and soft ground means that you do not distinguish between acceptable and unacceptable narrations. (al-sa‘b wa al-dhalul)
Another translation of this could be:
"You have begun to ride on both stubborn and obedient animals."
Whereas one should avoid a stubborn animal.
The purpose of this is also that narration should not be taken from every narrator, but only the narration of trustworthy and reliable individuals should be accepted, so that nothing is knowingly attributed to the Messenger of Allah (sallallahu alayhi wa sallam) which he did not actually say.