Imam Yahya ibn Sharaf al-Nawawi (1233–1277 CE) compiled 42 hadith (commonly called "40 Hadith") that form the foundation of Islam.
Hadith 1 — Intentions
"Actions are judged by intentions, and every man shall have only what he intended..." (Bukhari, Muslim)
This hadith is considered by scholars to be one-third of all Islamic knowledge — covering sincerity, intention, and reward.
Hadith 2 — Islam, Iman, Ihsan
The hadith of Jibril: "Islam is to testify there is no god but Allah... Iman is to believe in Allah, His angels, His books, His messengers, the Last Day, and divine decree... Ihsan is to worship Allah as if you see Him..."
Hadith 5 — Innovations
"Whoever introduces into this affair of ours anything that does not belong to it, it will be rejected." (Bukhari, Muslim)
Hadith 6 — Halal & Haram
"The halal is clear and the haram is clear, and between them are matters that are doubtful..."
Hadith 13 — Loving for Others
"None of you truly believes until he loves for his brother what he loves for himself."