Imam Bukhari (rahmatullahi 'alaih) narrates from Hadhrat 'ALI (radhiyallahu 'anhu) who said: "Narrate to the people that which they are acquainted with. Would you like that Allah Sub'hanahu wa Ta'ala and His Messenger Sallallahu 'alaihi wa sallam be rejected?" Adam ibn Abi Ilyas adds the words: "and leave out what they do not know" to this Hadith.
The meaning of "that which they are acquainted with" is that which they understand. And the meaning of "what they do not know" is that whose understanding causes them confusion. As for the words: "Would you like that Allah Sub'hanahu wa Ta'ala and His Messenger Sallallahu 'alaihi wa sallam be rejected?" - this means that if a person hears something that he does not understand and which he does not conceive to be possible, he will believe it to be impossible out of ignorance. He will therefore not believe in its existence. Then if something of this nature is reported from Rasulullah Sallallahu 'alaihi wa sallam, he will necessarily reject it. And rejecting the words of Rasulullah Sallallahu 'alaihi wa sallam entails rejecting Allah Sub'hanahu wa Ta'ala.
Al-Hafidh Ibn Hajar al-Asqalani (rahmatullahi 'alaih) says: "In this Hadith there is proof that something is confusing should not be mentioned before the masses. Similar is the opinion of the Hadhrat IBN MAS'UD (radhiyallahu 'anhu) when he says: 'No sooner you relate a Hadith to a people who cannot fathom it, it becomes a source of distress and tribulation for some of them.' (Narrated by Muslim in the introduction to his Sahih, vol. 1 p. 76)
"Among those who disliked narrating certain Traditions to the exclusion of others are: (1) Imam Ahmad ibn Hanbal (rahmatullahi 'alaih) with regard to the Traditions which outwardly show going against the leader, (2) Imam Malik (rahmatullahi 'alaih) with regard to the Traditions that mention the attributes [of Allah], i.e. those that outwardly portray similarity [to human attributes], (3) Imam Abu Yusuf (rahmatullahi 'alaih) with regard to Traditions in which particular words are mentioned (Gharibul Hadith), (4) The Sahabah/Companions ABU HURAYRAH, HUDHAYFAH (radhiyallahu 'anhumaa) and others who came before the previous three Imams also hold the same view."
"The general rule is that if a Hadith apparently supports an innovation, when its actual import is something else, then it is desirable to hold back such a Hadith from a person of whom it is feared that he may act on its outward meaning. And Allah Sub'hanahu wa Ta'ala Knows best. (Al-Hafidh Ibn Hajar (rahmatullahi 'alaih), Fath al-Bari, vol. 1 p. 225)
Bukhari and Muslim narrate on the authority of Anas Bin Malik Radhiyallahu 'anhu who said: Rasulullah Sallallahu 'alaihi wa sallam was on his camel and Mu'adh Bin Jabal Radhiyallahu 'anhu was sitting directly behind him. [Rasulullah Sallallahu 'alaihi wa sallam said to him] O Mu'adh! He replied: Here I am, O Rasulullah and I am pleased to be with you. Rasulullah Sallallahu 'alaihi wa sallam said: O Mu'adh! He replied: Here I am, O Rasulullah and I am pleased to be with you. Rasulullah Sallallahu 'alaihi wa sallam said: O Mu'adh! He replied: Here I am, O Rasulullah and I am pleased to be with you.Hafith Ibn Rajab says: "The 'ulamaa' say: 'It can be derived from this prohibition to Mu'adh Radhiyallahu 'anhu that he should not convey these glad-tidings lest the people become lax, that Traditions which contain concessions should not be mentioned openly among the masses so that they do not misunderstand the actual import of them. Mu'adh Radhiyallahu 'anhu heard such Traditions but these only served to increase his efforts in doing good deeds and in fearing Allah Sub'hanahu wa Ta'a'a. As for he who has not reached his rank, it is not far-fetched that he may become lax after relying on the outward meaning of this Hadith." ('Allama Shabbir Ahmad al-'Usmani, Fathul Mulhim, vol. 1, p.588)
Rasulullah Sallallahu 'alaihi wa sallam said: "When a person sincerely testifies that there is none worthy of worship but Allah and that Muhammad is His servant and Messenger, Allah Sub'hanahu wa Ta'ala makes him forbidden to the fire." [Mu'adh] said: "Should I not inform the people about this so that they may take glad tiding from this?" He replied: "No. Because if you do so, they will become lax." Mu'adh Radhiyallahu 'anhu eventually informed the people about this at the time of death fearing that if he didn't inform them, he would be committing a sin [of concealing knowledge].
Source: Excerpt from the book, Prophet Muhammad (saw) - The Teacher and his teaching methodologies by Shaikh Abdul-Fattah Abu Ghuddah (ra)