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There are some people who claim that certain individuals from the Salaf knew of more Ahadith than what we currently have, like Ahmad ibn Hanbal. They often go to extremes with this and say that if Ahmad said something that opposed a Sahih Hadith, it must mean that he knew a Hadith which we don't have today. This means that they essentially believe that some parts of the Wahy are lost.
This contradicts the words of Allah, for he has promised to preserve the Wahy, which means that every Ayah and every Hadith that has to do with our religion will remain preserved. Allah said:
"Verily we have sent down the Dhikr and we will surely guard/preserve it"
[Surah al-Hijr ayah 9]
The revelation is preserved with certainty, it is not surrounded by doubts. Meaning, it can't be that a text of the revelation is preserved but that its understanding is unknown. Nor can it be unable to determine with certainty whether it is truly from Allah and his Messenger or not. For this wouldn't be considered as "preservation", rather it'd be uncertainty and doubts, and something that is doubtful can never be truth. Allah said:
"...And indeed, Dhann (speculation, doubt, conjecture) avails not against the truth at all."
[Surah Al-Najm Ayah 28]
And Allah said:
"This is a book in which there is no doubt, a guidance to those who are
Muttaqīn..."
[Surah al-Baqarah ayah 2]
Thus we conclude that the revelation of Allah, i.e the Qur'an and Sunnah, is fully preserved, and so is its understanding since it is part of the revelation and must therefore also necessarily be preserved. There is no such case where someone from the Salaf knew something of the revelation which we don't have today.
والحمد لله رب العالمين