Bukhari

Hadith Commentary:
Abu Nu‘aym narrated in his Mustakhraj from Anas radi Allahu anhu that we were with ‘Umar radi Allahu anhu, and he was wearing a shirt patched in four places. At that moment, he recited this verse: (وَفَاكِهَةً وَأَبًّا)
Then he said: “We know what fākihah (fruit) is, but what is abb?”
Then he said: “We have been forbidden from unnecessary affectation (takalluf),” and he began to address himself, saying: “O son of the mother of ‘Umar! This is indeed affectation. If you do not know what abb is, what harm is there?”

Hadith Commentary:
➊
The background of this hadith is that Anas radi Allahu anhu narrates that we were sitting with Umar radi Allahu anhu, and he was wearing a shirt on the back of which there were four patches. During this, he recited this verse:
(وَفَاكِهَةً وَأَبًّا)
Then he said:
As for (فَاكِهَةً),
we know what it is, but (أَبًّا),
what is it? Then he said:
We have been forbidden from unnecessary affectation (takalluf).
➋
In another narration, Umar radi Allahu anhu, addressing himself, said:
“O Umar radi Allahu anhu! This itself is affectation (takalluf). If you do not know what (أَبًّا)
is, then what difference does it make?” (Fath al-Bari: 13/332)
Ibn Abbas radi Allahu anhu explained the meaning of this word to Umar radi Allahu anhu, saying that it means fodder for animals.
Because of these exegetical points, Umar radi Allahu anhu used to give him a place close to himself.
(Fath al-Bari: 13/333)
Note:
Imam Bukhari rahimahullah’s purpose in this is that the statement of a Companion (أَمْرِنَا)
and (نُهِيْنَا)
is considered as having the ruling of a marfu‘ hadith (attributed to the Prophet) even if the mention of the Messenger of Allah sallallahu alayhi wa sallam is not present with it. That is why he sufficed in this hadith with (نُهِينَا عَنِ التَّكَلُّفِ)
and omitted the rest of the incident.
(Fath al-Bari: 13/333)
The purpose is that unnecessary affectations are not permitted in the Shari‘ah, as Imam Bukhari rahimahullah has established a chapter regarding this.