Are the statement and actions of the pious predecessors proof?
No! They are not a proof Allah - The Glorified and Exalted - says:
اتبعوا ما أُنْزِلَ إليكم من ربكم و لا تتبعوا من دونه أولياء قليلاً ما تذكرون
Follow what has been revealed to you from your Lord and do not follow awliyaa besides Him. Little do you remember. [Al-A’raf: 3]
The people, as it pertains to this subject, are between being excessive and (between) negligence, i.e. with regards to the significance of the statements of the salaf. From them you have he who disregards their statements and says, “They are men and we are men!” Correct they are men and we are men, however the difference between us and them is like the difference between the sky and the earth, as we seek assistance in their understanding as it pertains to understanding Allah’s book and the sunnah of Allah’s Messenger ﷺ.
Az-Zuhri had a companion, both of them would write the narrations of Allah’s Messenger ﷺ but when they concluded (writing) what reached them from the narrations of Allah’s Messenger ﷺ he (Az-Zuhri) wrote the statements of the companions, while his companion did not believe it to be knowledge. Thus his companion said, “He (Az-Zuhri) prospered and was successful” or similar (to that) in meaning.
Therefore if there was no benefit in them the scholars would neither have compiled them nor quoted them in their authored works, e.g. Ibn Jarir in his tafsir, Ibn Abi Shaiba in his musannaf, Abdur Razzaq in his musannaf, Al-Baihaqi in his sunan, and in an abundance of sunan and in other than them wherein they (the scholars) quote the statements of the salaf. So it is not said that they (the salaf’s statements) have no value, on the contrary we benefit from their understanding, and it is incumbent that we understand Allah’s book and the sunnah of His Messenger ﷺ according to their understanding.
As for it being a proof then no, Allah - The Glorified and Exalted - says:
و ما اختلفْتم فيه من شيءٍ فحكمه إلى الله
And in whatsoever you differ, the decision is with Allah.. [Ash-Shura: 10]
And He says:
فئن تنازعتم في شيءٍ فردُّوه إلى الله و الرسول
So if you differ in any matter, refer it to Allah and the Messenger… [An-Nisaa: 59]
وأن هذا صراطي مستقيمًا فاتبعوه
And this is my straight path so follow it… [Al-An’am: 153]
He also says:
وما آتاكم الرسول فخذوه
Whatsoever the Messenger gives you, take it… [Al-Hashr: 7]
As for the third category, as we have just mentioned two categories i.e. (one of them being) rejecting the statements of the companions and the taabi’een and saying “They are men and we are men” (said) specifically (by) the takfiris. And the other category that exceed limits with regards to it (the salaf’s statements), to justify by usage of it (opinions and positions), and to make it a proof, this is a mistake due to that which you’ve heard from evidence.
The third category says, “We seek assistance from Allah - The Glorified and Exalted - then with the understanding of our pious predecessors in understanding the book and the sunnah.”