The hadeeth about one who memorizes forty ahaadeeth is a weak hadeeth
1 – This hadeeth is da’eef (weak). Despite its large number of isnaads they do not support one another, and there are many different versions with different wordings, such as, “…Allaah will resurrect him on the Day of Resurrection among the fuqaha’ and scholars,” and “… Allaah will resurrect him a scholar and a faqeeh,” and “I will be an intercessor and a witness for him on the Day of Resurrection,” etc.
(a) al-Haafiz ibn Hajar said:
It was narrated by al-Hasan ibn Sufyaan in his Musnad and in his Arba’een, from the hadeeth of Ibn ‘Abbaas, and he narrated it from thirteen of the Sahaabah. It was narrated by Ibn al-Jawzi in al-‘Ilal al-Mutanaahiyah, and he explained that all of these ahaadeeth are da’eef. Ibn al-Mundhir discussed it in a separate volume, and I discussed it in the sixteenth chapter of al-Imla’. I mentioned all of its isnaads in one volume but there is not one isnaad that is free of faults.
Al-Talkhees al-Habeer, 3/93, 94.
(b) Ibn al-Mulaqqin said:
The hadeeth “Whoever preserves forty hadeeth for my ummah will be recorded as a faqeeh” was narrated via approximately twenty isnaads, all of which are da’eef (weak). Al-Daaraqutni said: All its isnaads are da’eef and none of them are proven. Al-Bayhaqi said: Its isnaads are weak.
Khalaasat al-Badr al-Muneer, 2/145
This text is well known among the people but it has no saheeh isnaad.
Shu’ab al-Eemaan, 2/270
The scholars of hadeeth are agreed that it is a weak hadeeth, although it has many narrations.
Muqaddimat al-Arba’een al-Nawawiyyah.
2 – We should note here that there are reports in the Sunnah which mention the virtue of listening to the hadeeth of the Prophet (peace and blessings of Allaah be upon him) and conveying it, even if it is just one hadeeth.
It was narrated that Zayd ibn Thaabit said: I heard the Messenger of Allaah(peace and blessings of Allaah be upon him) say, “May Allaah bless a man who hears a hadeeth from us and memorizes it so that he can convey it to others, for perhaps he is conveying it to one who will understand it better than him, and perhaps the one who conveys knowledge does not understand it himself.”
(Narrated and classed as hasan by al-Tirmidhi, 2656; also narrated by Abu Dawood. 3660; Ibn Maajah, 230).
What is meant is that Allaah has singled him out for happiness and joy because of the knowledge with which He has blessed him and the status among people in this world and blessings in the Hereafter that he attains thereby, so that the joy and blessings are seen on his face. And it was suggested that the hadeeth indicates what happens to the person of joy and blessing that appears on his face.
Tuhfat al-Ahwadhi, 7/347, 348
And Allaah knows best.