All the Hadith are not present in the Sihah Sittah (the 6 most rigorously authenticated collections of Hadith). Thus, Shaykh Abul Faidh Muhammad bin Ali (May Allah shower His Mercy on him) has written in Jawahir Ul Usool that, not counting the repetitions, there are 4000 Hadith in Sahih Bukhari and Sahih Muslim both.
Shah ‘Abdul ‘Aziz (May Allah shower His Mercy on him) has written in Bustan Ul Muhadditheen that there are 4800 Hadith in Sunan Abu Dawood. These include many repeated Hadith as well and those Hadith which are related in Sahih Bukhari and Sahih Muslim as well.
Other books of Hadith have many Hadith which are in these 3 books of Hadith. Thus, if enumerated all the Hadith of the Sihah will not be more than 10,000 to 12,000. Imam Qustullani (May Allah shower His Mercy on him) has related the words of Imam Bukhari in his commentary on Sahih Bukhari: I know 1,00,000 rigorously authenticated Hadith.
In Jawahir Ul Usool, the words of Imam Ahmed bin Hambal (May Allah be well pleased with him) are related: More than 7,50,000 Hadith are rigorously authenticated.
Just see! If one insists that only those Hadith which are present in the Sihah are rigorously authenticated, then thousands of Hadith will be made invalid and compilation of those books will be considered to be an exercise in futility, although the Hadith-experts will never do anything wasteful or useless. Scholars know that great Hadith-experts like Imam Ibn Hajr ‘Asqalani (May Allah shower His Mercy on him) and others frequently use Hadith from other books to deduce rulings and commands.
Thus, insisting on Hadith from the Sihah will be a needless burden. In fact, it will turn out to be an objection on the great Hadith-experts like Imam Bukhari, etc. as to why they didn’t record each single Hadith of the thousands they remembered. We cannot think that these persons were miserly in sharing the knowledge of Hadith. The truth is that each Hadith-expert had an aim, an objective in the writing of that particular book. Nobody’s objective was to write down every Hadith they knew. Otherwise, they would have claimed that other than their books, all other Hadith are fabricated or weakly authenticated.
[Excerpted from Anwaar-e-Ahmadi]