I always understood Riyad As-Saliheen (the famous book compilation of hadith) to contain ahadith from Saheeh Bukhari or Saheeh Muslim, or both. But when perusing it on sunnah.com, I noticed one hadith says at the end:
[Abu Dawud and At- Tirmidhi].
What exactly is Riyad As-Saliheen? Where do the ahadith come from? What range are they (eg. is it a collection of only saheeh, or does it also contain hassan, da'eef, and mawdoo' ahadith?)
As you mentioned, Riyad-us-Saliheen is compilation of hadiths by Imam An-Nawawi, a great scholar of Hadith and fiqh. He is known for such compilations like 40 Hadeeth by An-Nawawi, 40 Hadith Qudsi etc. Riyad-us-Saliheen is a popular and useful compilation because of its classification based on wide range of topics. It serves as a hadith primer. It has selected hadiths of approximately 2000 hadiths from the six major collections: Bukhari, Muslim, Abu Dawood, Tirmidhi, An-Nasa'i and Ibn Majah. These 2000 hadiths are grouped in 372 Chapters and 19 Sections with titles - The Book of Good Manners, The Book about the Etiquette of Eating, The Book of Dress, The Book of the Etiquette of Sleeping, Lying and Sitting, etc and so on. Hence, it serves as an easy reader.
Since the hadiths compiled are from the six major collections, it can be considered or called as authentic. However, it is not devoid of weak hadiths. There are some hadiths checked and declared weak by Shaykh Al-Albaani
