Imam Nawawi's Forty Hadith and Forty Hadith Qudsi are widely regarded as the most popular anthology and the best introduction to the study of the Prophet's Sayings in the English language.
The hadith summarize the essential teachings of Islam, concentrating on the following areas: affirmation of the doctrine of tawhid and understanding Allah (swt) in all His attributes, the significance of the five pillars of Islam, attainment of virtue, self-sacrifice for the cause of Allah (swt) and preparation for the day of judgement.
The Hadith Qudsi, or sacred hadith, have a particularly important status because their meaning is entirely from Allah (swt) while the wording is that of the Prophet Muhammad (s) himself. For each of the eighty hadith presented, the Arabic original has been printed alongside the English translation, which has earned a reputation for its scholastic accuracy and readability
Among the many definitions given by the early scholars to Sacred Hadith is that of as-Sayyid ash-Sharif al-Jurjani (died in 816 A.H.) in his lexicon At-Tarifat where he says: "A Sacred Hadith is, as to the meaning, from Allah the Almighty; as to the wording, it is from the messenger of Allah (peace and blessings of Allah be upon him). It is that which Allah the Almighty has communicated to His Prophet through revelation or in dream, and he, peace be upon him, has communicated it in his own words."
The hadith qudsi are the sayings of the Prophet divinely communicated to him. The present collection has been compiled from all the available books of hadith. The forty chosen here are all well authenticated and present many of the doctrinal, devotional and ethical elements of Islam.
[The Above taken from : An introduction to the subject and select sample texts, see e.g. Ibrahim Izzuddin and Denis Johnson-Davies: Forty Hadith Qudsi, Beirut, Damascus, 1980.]
While the common factor between hadith qudsi and the Qur'an is that both contain words from Allah which have been revealed to Muhammad, the main points of difference between Qur'an and hadith qudsi are as follows: * In the Qur'an the precise wording is from Allah, while in the hadith qudsi the wording is given by the Prophet Muhammad. * The Qur'an has been brought to Muhammad only by the Angel Gabriel, while hadith qudsi may also have been inspired otherwise, such as e.g. in a dream. * The Qur'an is inimitable and unique, but not so the hadith qudsi. * The Qur'an has been transmitted by numerous persons, (tawatur) but the hadith and hadith qudsi often only by a few or even one individual. There are hadith qudsi which are sahth, but also others hasan, or even da'if, while there is no doubt at all about any aya from the Qur'an. * Another point is that a hadith qudsi cannot be recited in prayer.