May a later scholar declare a hadith sound that the earlier imams did not?

In the second point of the first type, Ibn al-Salah writes: when we find, in the hadith booklets or elsewhere, a hadith whose chain is sound, which we do not find in either of the two Sahihs and which no recognised, widely known work of the hadith imams declares sound, we do not venture to affirm that it is sound. ^{1 2}

His reason is that in these ages every chain includes men who relied on what was in their books, without the memory, precision and mastery the sound requires. Knowledge of the sound and the good has therefore come to rest on the imams' works, whose fame keeps them safe from alteration and corruption. The main purpose of the chains still in circulation has become to keep alive the chain of transmission, a distinction of this community. ²

The editor, Nur al-Din Itr, considers it more cautious to say "sound in chain" rather than declare a hadith sound outright, since it may have a defect hidden from the one examining it. ²