Praise be to Allah.
1 – This hadeeth is da’eef (weak). Despite its large number of isnads (chains of transmission) they do not support one another, and there are many different versions with different wordings, such as, “…Allah will resurrect him on the Day of Resurrection among the fuqaha (jurists) and scholars,” and “… Allah will resurrect him a scholar and a faqeeh (jurist),” and “I will be an intercessor and a witness for him on the Day of Resurrection,” etc.
(a) al-Hafiz ibn Hajar said:
It was narrated by al-Hasan ibn Sufyan in his Musnad and in his Arba’een, from the hadeeth of Ibn ‘Abbas (may Allah be pleased with him), and he narrated it from thirteen of the Sahabah/Companions (may Allah be pleased with them). It was narrated by Ibn al-Jawzi in al-‘Ilal al-Mutanahiyah, and he explained that all of these hadeeths 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 isnads in one volume but there is not one isnad 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 (followers) will be recorded as a faqeeh” was narrated via approximately twenty isnads, all of which are da’eef (weak). Al-Daraqutni said: All its isnads are da’eef and none of them are proven. Al-Bayhaqi said: Its isnads are weak.
Khalasat al-Badr al-Muneer, 2/145
This text is well known among the people but it has no saheeh (authentic) isnad.
Shu’ab al-Eeman, 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 Allah be upon him) and conveying it, even if it is just one hadeeth.
It was narrated that Zayd ibn Thabit (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, “May Allah 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 (sound) by al-Tirmidhi, 2656; also narrated by Abu Dawood. 3660; Ibn Majah, 230).
What is meant is that Allah 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 Allah knows best.