I bought book of Imam Nesai "'amelu leyli ua nahar" and wanted to memorize many du'as.But according to book some of hadises sahih, hasen, daif, garib and some of them are included in 2,3,4 Imams book of Kutubusitte but there is no information sahih or not. They used Suyuti, Ibn Hacer, Darekutni and other big hadis alims as a reference. What criteria I have to follow which duas I can memorize? I afraid that I will memorize one amal, dua, but in reality Prophet (saw) didn't do that amal.. So it will be bid'a?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Firstly, we say that there is no doubt that ‘Amal Al-Yawm wal-Laylah by Imaam Abu ‘Abd Ar-Rahmaan Ahmad ibn Shu‘ayb An-Nasaa’i, author of the renowned Sunan of An-Nasaa’i (d. 303 AH), is a beneficial book and its author was one of the leading Hadeeth scholars. The Ahaadeeth presented in the book include Saheeh (sound), Hasan (good) or Dha‘eef (weak) Ahaadeeth. Dr. Farooq Hamaadah underlined in his revision of the book that the most that can be said about its weakness is that some of the narrators are Majhool Al-Haal (whose trustworthiness cannot be affirmed) or Majhool Al-‘Ayn (cannot be identified from any of the available biographical sources; his identity is unknown given the lack of precise information about him).

It is well-known that it is permissible to act in accordance with a Dha‘eef Hadeeth in matters pertaining to virtuous deeds as long as it is not extremely weak. Imaam An-Nawawi said, summarizing the standard rule on using weak Ahaadeeth: "Scholars agreed that a weak Hadeeth may be used in matters pertaining to virtuous deeds, but may not be used as primary evidence in determining what is Halaal and Haraam (lawful and unlawful)." [Al-Majmoo’]

Al-Khateeb Ash-Shirbeeni wrote: "The conditions for acting in accordance with a weak Hadeeth in relation to virtuous deeds are that: it should not be extremely weak; it should fall under a general religious principle already present (i.e. should have basis in the Sharee‘ah); and the one acting in accordance with it should not believe that the action is part of Sunnah (i.e. should not attribute it to the Prophet, sallallaahu ‘alayhi wa sallam) .” [Mughni Al-Muhtaaj]

For more benefit, please refer to Fataawa 13202 and 166199.

Therefore, you are advised to memorize the Ahaadeeth in this book, even the weak ones. There is no harm in memorizing such Ahaadeeth or in acting in accordance with weak Ahaadeeth that the (above-named) editor has not described as extremely weak.

Allaah Knows best.