Imam Nawawee said in his book Riyaadh Saliheen
Chapter: The dislike of walking in one footwear or leather socks without necessity and the dislike of wearing footwear and leather socks while standing without reason.
Abu Hurayrah (may Allaah be pleased with him) reported that the prophet (commendations and peace of Allaah be upon him) said:
None of you should walk in one footwear. He should wear both of them or pull both of them”
In another report
Or he should leave (both of) them bare together
– Agreed upon.
He (Abu Hurayrah) also said: I heard the messenger of Allaah (commendations and peace of Allaah be upon him) saying:
If the footwear thong of one of you cuts, he shouldn’t walk in the other until he fixes it
-Reported by Muslim
And Jabir (may Allaah be pleased with him) reported that the messenger of Allaah (commendations and peace of Allaah be upon him)
Forbade that a person wears his footwear while standing.
-Compiled by Abu Dawood with a good chain.
Sheikh Uthaymeen’s Commentary
These hadiths are about footwear and the dislike of wearing footwear or leather socks on one leg. Rather he should either leave both of them bare. Meaning that he should not wear anything on both legs, or he should wear footwear on both.
It should be known that wearing footwear is a part of the sunnah so also being bare (footed) is also part of sunnah. This is why the prophet prohibited excessive luxury and he commanded being bare footed sometimes.
So the sunnah is that it is not bad for a person to wear slippers, so also it is befitting for him to (sometimes) walk barefooted among people to show this sunnah that some people condemn.
If they see someone walking barefooted they say, ‘what is this? This is one of the ignorant (people)’. But this is an error. Because the prophet (commendations and peace of Allaah be upon him) prohibited excessive luxury and commanded bare footedness sometimes.
Also, when wearing footwear:
Start with the right when putting-on and with the left when putting off.
If you have your footwear on and you wish to enter the mosque with your footwear, Inspect it well before entering. If there is impurity or filth (beneath it) then rub it on the floor until it (the impurity or filth) disappears then pray in them.
Indeed this is part of the sunnah. The prophet (commendations and peace of Allaah be upon him) said:
Oppose the Jews, pray in your footwear
This is because the Jews do not pray in their footwear.
The sunnah is for him to pray in his footwear just as many people pray in their khuff (leather socks). There is no difference between the khuff and slippers. People detest it because it is a sunnah that has been killed.
However, this (applies) if the mosques are laid with what the mosques of old were laid with. The mosques in the past used to be laid with stones, pebbles, desert sand or similar materials. So footwear couldn’t damage (those floors). But now that mosques have been laid, if people entered the mosques with their footwear they would surely and undoubtedly stain the mosque. This is because most of the people do not care, he will enter while there is filth or impurity.
This is why today the scholars believe that a person shouldn’t enter the mosque with his footwear based on the fact that they are laid with rugs that will get dirty if someone entered with his footwear.
Therefore if a person wishes to practice this sunnah he should observe the tahajjud prayer or voluntary prayers in his house with his footwear. That way he would have followed the commandment of the prophet (commendations and peace of Allaah be upon him) in his statement:
Indeed the Jews do not pray in their footwear.
(Abu Daawood: 652)
Then he (referring to Imam Nawawee) mentioned some hadiths including the hadith of Abu Hurayrah that the prophet (commendations and peace of Allah be upon him) forbade that a person wears one footwear. Meaning that he should either wear the pair of footwear together or he should remove the pair together. But wearing one (leg) and leaving the other, this has been prohibited.
The reason for that, and Allaah knows best, is that this religion of Islam has come with justice even in clothing. Don’t wear footwear on one leg and leave the other bare. Because there is injustice in this to the other leg that you didn’t wear footwear on.
That is why the prophet (commendations and peace of Allaah be upon him) prohibited walking in one footwear.
The scholars say: even if it is to fix the other leg. Rather wait till you fix the other then you wear them. As it is in the second hadith of Abu Hurayrah:
When the (sandal) thong of one of you cuts, he shouldn’t wear it until he has fixed the other, Then (after he has fixed the other) he should wear them together.
As for the hadith of Jabir (may Allaah be pleased with him) which Abu Daawood reported that the prophet (commendations and peace of Allaah be upon him) prohibited that one wears his footwear while standing.
This is regarding footwear that requires adjustment while wearing. Because if a person wears his footwear -that requires adjustment- while standing he might fall when he lifts his leg to adjust the footwear. He might fall on the back of his head or some of his privacy might be exposed and he would be harmed.
But there no problem with the common slippers if a person wears it while standing. It isn’t included in the prohibition. Because the common slippers are easy to pull off and wear.
And Allaah is the granter of success.
 Saheehul Bukhaaree (5855) and Saheeh Muslim (2098)
 Saheeh Muslim (2098)
 Sunan Abu Daawood (4135) and authenticated by Sheikh Albany in Saheeh Sunan Abee Daawood (3483)
 Sharh Riyadh Saliheen by Sheikhul Uthaymeen
 Reported by Abu Daawood (4160) and authenticated by Sheih Albany in Saheeh Abu Daawood.
 Sunan Abee Daawood: 652. Authenticated by Sheikh Albaany