Hadith: When anyone of you comes to the mosque, he should look (at his sandals). If he sees any filth or dirt on his sandals, he should wipe it off and pray in them

عن أبي سعيد الخُدْري رضي الله عنه قال: بَينما رسول الله صلى الله عليه وسلم يصلِّي بأصحابه إذ خَلع نَعْلَيه فوضَعَهُما عن يساره، فلمَّا رأى ذلك القوم أَلْقَوْا نِعَالَهُمْ، فلمَّا قَضى رسول الله صلى الله عليه وسلم صلاته، قال: «ما حَمَلَكُمْ على إِلقَاءِ نِعَالِكُم»، قالوا: رأيْنَاك أَلقَيْت نَعْلَيْك فَأَلْقَيْنَا نِعَالَنَا، فقال رسول الله صلى الله عليه وسلم : إن جبريل صلى الله عليه وسلم أتَانِي فأخْبَرنِي أن فيهما قَذَرًا -أو قال: أَذًى-، وقال: إذا جاء أحدكم إلى المسجد فلينظر: فإن رَأى في نَعْلَيه قَذَرا أو أَذًى فَلْيَمْسَحْه وليُصَلِّ فيهما.
[صحيح] - [رواه أبو داود وأحمد والدارمي]

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported: Once the Messenger of Allah (may Allah's peace and blessings be upon him) was leading his Companions in prayer when all of a sudden he removed his sandals and placed them on his left. When the people saw that, they threw off their sandals too. When the Messenger of Allah (may Allah's peace and blessings be upon him) finished his prayer he said: "What made you all throw off your sandals?" They said: "We saw you throw off your sandals, so we threw off ours." The Messenger of Allah (may Allah's peace and blessings be upon him) said: "Jibrīl (may Allah's peace and blessings be upon him) came to me and told me that they had some filth on them (or he said 'dirt')." Then the Prophet (may Allah's peace and blessings be upon him) said: "When anyone of you comes to the mosque, he should look (at his sandals). If he sees any filth or dirt on his sandals, he should wipe it off and pray in them."
Sahih/Authentic. - [Abu Dawood]

Explanation

One day, the Prophet (may Allah's peace and blessings be upon him) led his Companions in prayer, then he suddenly removed his sandals and placed them on his left. When the Companions (may Allah be pleased with him) saw this, they removed their sandals, following the example of the Prophet (may Allah's peace and blessings be upon him). When he finished his prayer and turned towards them, he saw them in a state that they had not been in when they began the prayer. So, he asked them about the reason why they removed their sandals. They responded with their statement: "We saw you throw off your sandals, so we threw off ours," i.e. following your attitude. They did this because they thought that the permissibility of praying in sandals had been abrogated. So, the Prophet (may Allah's peace and blessings be upon him) explained to them why he removed his sandals as he said: "Jibrīl (may Allah's peace and blessings be upon him) came to me and told me that they had some filth on them (or he said 'dirt')," meaning that Jibrīl (peace be upon him) came to the Prophet (may Allah's peace and blessings be upon him) while he was praying and informed him that his sandals were not suitable to pray in because there was filth or dirt on them (the sub-narrator was uncertain which of the two words was used). Then, he said: "When anyone of you comes to the mosque, he should look (at his sandals). If he sees any filth or dirt on his sandals he should wipe it off and pray in them." Meaning that when a person wants to enter the mosque and pray while wearing his sandals, he must check them first. If he sees any filth or dirt on them he must wipe it off by means of rubbing them against the ground or anything else in order to remove it, and then he can pray in them. The Hadīth, however, does not mean that a person must pray in his sandals, rather, he may or may not pray in them. But if he wishes to pray in them, he must be sure of their cleanliness, and then he can pray in them if he likes. Whoever is praying and does not notice an impurity, whether on his sandals, head covering, hat or clothes, and then realizes it later while he is praying, he must quickly remove the soiled item. If filth is on his garment or pants and he is able to remove them without exposing his ‘Awrah, he should remove them. Then, he should continue with his prayer without having to repeat it. If he is unable to remove the soiled item except by having his ‘Awrah exposed, he must leave the prayer, (remove the dirt), cover his ‘Awrah, and repeat the prayer.