Shaikh Moulana Muhammad Zakariyya (Allah have mercy on him) once mentioned:
Imaam Abu Hanifah (Allah have mercy on him) selected five Ahaadith from five hundred thousand Ahaadith. These five Ahaadith are such that through practising on them, one will be practising on the entire Deen.
After Imaam Abu Hanifah, Imaam Abu Dawood (Allah have mercy on them) selected four thousand eight hundred Ahaadith from five hundred thousand Ahaadith and compiled them into his book, Sunan Abi Dawood. From these Ahaadith four thousand eight hundred Ahaadith, he selected four Ahaadith and stated that the one who practises on them will be practising upon the entire Deen. All these four Ahaadith are among the five Ahaadith that Imaam Abu Hanifah had selected. Imaam Abu Dawood did not select the fifth Hadith as he understood that the meaning of the fifth Hadith can be included in the other four Ahaadith.
Imaam Abu Hanifah passed away in the year 150 A.H.
Imaam Abu Dawood was born fifty two years later, in the year 202 A.H. From this, it seems possible that Imaam Abu Dawood had taken this view from Imaam Abu Hanifah.
These four Ahaadith are,
1. The acceptance of) all actions are based on the intentions (with which the actions were carried out).
2. One will not be a true believer until he wishes for his brother that which he wishes for himself.
3. From the beauty of a person’s Islam is that he leaves out that which does not concern him.
4. Permissible (halal) is clear and impermissible (haraam) is clear, and between them are such things which are doubtful and many people do not know it. The one who abstains from these doubtful things will protect his Deen and his honor.
(Suhbat Baa Awliyaa pg. 94-97)