The Imam questioning his companions in order to test their knowledge

The Prophet (ﷺ) said, "Amongst the trees, there is a tree, the leaves of which do not fall and is like a Muslim. Tell me the name of that tree." Everybody started thinking about the trees of the desert areas. And I thought of the date-palm tree. The others then asked, "Please inform us what is that tree, O Allah's Messenger (ﷺ)?" He replied, "It is the date-palm tree."

حَدَّثَنَا خَالِدُ بْنُ مَخْلَدٍ، حَدَّثَنَا سُلَيْمَانُ، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ دِينَارٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ إِنَّ مِنَ الشَّجَرِ شَجَرَةً لاَ يَسْقُطُ وَرَقُهَا، وَإِنَّهَا مَثَلُ الْمُسْلِمِ، حَدِّثُونِي مَا هِيَ ‏"‏‏.‏ قَالَ فَوَقَعَ النَّاسُ فِي شَجَرِ الْبَوَادِي‏.‏ قَالَ عَبْدُ اللَّهِ فَوَقَعَ فِي نَفْسِي أَنَّهَا النَّخْلَةُ، ثُمَّ قَالُوا حَدِّثْنَا 

مَا هِيَ يَا رَسُولَ اللَّهِ قَالَ ‏"‏ هِيَ النَّخْلَةُ ‏"‏‏.‏

Reference	: Sahih al-Bukhari 62
In-book reference	: Book 3, Hadith 4
USC-MSA web (English) reference	: Vol. 1, Book 3, Hadith 59
(deprecated numbering scheme)