The first bridge on Judgment Day — called as-Sirāt (Arabic: الصراط) — is mentioned in a long hadith in Sahih al-Bukhari 97/65 that details a lot of events that will take place then as narrated on the authority of Abu Sa'īd al-Khudri when the companions asked the Prophet ﷺ about Judgment Day. In this hadith (partially quoted below), as-Sirāt is referred to as a bridge over Hellfire:
ثُمَّ يُؤْتَى بِالْجَسْرِ فَيُجْعَلُ بَيْنَ ظَهْرَىْ جَهَنَّمَ ". قُلْنَا يَا aرَسُولَ اللَّهِ وَمَا الْجَسْرُ قَالَ " مَدْحَضَةٌ مَزِلَّةٌ، عَلَيْهِ خَطَاطِيفُ وَكَلاَلِيبُ وَحَسَكَةٌ مُفَلْطَحَةٌ، لَهَا شَوْكَةٌ عُقَيْفَاءُ تَكُونُ بِنَجْدٍ يُقَالُ لَهَا السَّعْدَانُ، الْمُؤْمِنُ عَلَيْهَا كَالطَّرْفِ وَكَالْبَرْقِ وَكَالرِّيحِ وَكَأَجَاوِيدِ الْخَيْلِ وَالرِّكَابِ، فَنَاجٍ مُسَلَّمٌ وَنَاجٍ مَخْدُوشٌ وَمَكْدُوسٌ فِي نَارِ جَهَنَّمَ، حَتَّى يَمُرَّ آخِرُهُمْ يُسْحَبُ سَحْبًا
"Then the bridge will be laid across Hell." We, the companions of the Prophet (ﷺ) said, "O Allah's Messenger (ﷺ)! What is the bridge?" He said, "It is a slippery (bridge) on which there are clamps and (Hooks like) a thorny seed that is wide at one side and narrow at the other and has thorns with bent ends. Such a thorny seed is found in Najd and is called As-Sa'dan. Some of the believers will cross the bridge as quickly as the wink of an eye, some others as quick as lightning, a strong wind, fast horses or she-camels. So some will be safe without any harm; some will be safe after receiving some scratches, and some will fall down into Hell (Fire). The last person will cross by being dragged (over the bridge)."
— Sahih al-Bukhari, Book 97, Hadith 65
The first bridge was called as-Sirāt by the Prophet ﷺ and further described in another long hadith narrated on the authority of Abi Hurairah in Sahih al-Bukhari 10/201 when the companions asked the Prophet ﷺ if they would see Allah ﷻ on Judgment Day. In this hadith (partially quoted below) as-Sirāt is named and described:
فَيَأْتِيهِمُ اللَّهُ فَيَقُولُ أَنَا رَبُّكُمْ. فَيَقُولُونَ أَنْتَ رَبُّنَا. فَيَدْعُوهُمْ فَيُضْرَبُ الصِّرَاطُ بَيْنَ ظَهْرَانَىْ جَهَنَّمَ، فَأَكُونُ أَوَّلَ مَنْ يَجُوزُ مِنَ الرُّسُلِ بِأُمَّتِهِ، وَلاَ يَتَكَلَّمُ يَوْمَئِذٍ أَحَدٌ إِلاَّ الرُّسُلُ، وَكَلاَمُ الرُّسُلِ يَوْمَئِذٍ اللَّهُمَّ سَلِّمْ سَلِّمْ. وَفِي جَهَنَّمَ كَلاَلِيبُ مِثْلُ شَوْكِ السَّعْدَانِ، هَلْ رَأَيْتُمْ شَوْكَ السَّعْدَانِ. قالوا: نَعَمْ. قال: فَإِنَّهَا مِثْلُ شَوْكِ السَّعْدَانِ، غَيْرَ أَنَّهُ لاَ يَعْلَمُ قَدْرَ عِظَمِهَا إِلاَّ اللَّهُ، تَخْطَفُ النَّاسَ بِأَعْمَالِهِمْ، فَمِنْهُمْ مَنْ يُوبَقُ بِعَمَلِهِ، وَمِنْهُمْ مَنْ يُخَرْدَلُ ثُمَّ يَنْجُو
Then Allah will come to them again and say, 'I am your Lord.' They will say, 'You are our Lord.' Allah will call them, and As-Sirat (a bridge) will be laid across Hell and I (Muhammad) shall be the first amongst the Apostles to cross it with my followers. Nobody except the Apostles will then be able to speak and they will be saying then, 'O Allah! Save us. O Allah Save us.' There will be hooks like the thorns of Sa'dan [??] in Hell. Have you seen the thorns of Sa'dan [??]?" The people said, "Yes." He said, "These hooks will be like the thorns of Sa'dan [??] but nobody except Allah knows their greatness in size and these will entangle the people according to their deeds; some of them will fall and stay in Hell forever; others will receive punishment (torn into small pieces) and will get out of Hell.
— Sahih al-Bukhari, Book 10, Hadith 201
The second bridge — called al-Qantara (Arabic: القنطرة) in Arabic — is where the second qissās (retaliation) takes place on Judgment Day. The first qissās is often called a general qissās and it is retaliatory in the sense that exchange of good deeds and bad deeds between the opponents takes place as a form of compensation, and that it is among all creations, believers, disbelievers, animals, etc. (see Sahih Muslim 45/77). The second qissās is often called a special qissās as it is between believers only and in specific the ones that will end up being in Paradise. We are informed of this second bridge through the hadiths:
عن أبي سعيد الخدري رضي الله عنه قال قال رسول الله صلى الله عليه وسلم: يَخْلُصُ الْمُؤْمِنُونَ مِنَ النَّارِ، فَيُحْبَسُونَ عَلَى قَنْطَرَةٍ بَيْنَ الْجَنَّةِ وَالنَّارِ، فَيُقَصُّ لِبَعْضِهِمْ مِنْ بَعْضٍ، مَظَالِمُ كَانَتْ بَيْنَهُمْ فِي الدُّنْيَا، حَتَّى إِذَا هُذِّبُوا وَنُقُّوا أُذِنَ لَهُمْ فِي دُخُولِ الْجَنَّةِ، فَوَالَّذِي نَفْسُ مُحَمَّدٍ بِيَدِهِ لأَحَدُهُمْ أَهْدَى بِمَنْزِلِهِ فِي الْجَنَّةِ مِنْهُ بِمَنْزِلِهِ كَانَ فِي الدُّنْيَا
Narrated Abu Sa'id Al-Khudri: Allah's Messenger (ﷺ) said, "The believers, after being saved from the (Hell) Fire, will be stopped at a bridge between Paradise and Hell and mutual retaliation will be established among them regarding wrongs they have committed in the world against one another. After they are cleansed and purified (through the retaliation), they will be admitted into Paradise; and by Him in Whose Hand Muhammad's soul is, every one of them will know his dwelling in Paradise better than he knew his dwelling in this world."
— Sahih al-Bukhari, Book 81, Hadith 124
There are many books and commentaries on al-Qantara. In this answer, I referred to the research done by Amīn ibn 'Abdullah Ash-Shiqawi, Ph.D. in his book Ad-Durrar al-Muntaqāh min al-Kalimāt al-Mulqāh, and in specific the chapter on "In between Paradise and Hellfire". Dr. Ash-Shiqawi summarized some of the commentaries made by scholars as follows:
- Al-Mu'jam al-Wassīt (371). Linguistically, al-Qantara is an arched bridge over a river for crossing over from one side to the other.
- Fat'h al-Bāri (11/399). Ibn Hajar said that it may be that the people [on al-Qantara] are the ones on the elevated partition [between Paradise and Hellfire] that their good deeds and bad deeds are equal (see Qur'an 7:46). He added that scholars do not agree whether al-Qantara is attached to as-Sirāt or a separate construction and that Al-Qurtubi chose that it was a separate construction.
- Al-Bidāyah wa an-Nihāyah (20/101). Ibn Kathir said that al-Qantara is after the believers cross as-Sirāt over Hellfire, and it may be constructed over another form of punishment device that only Allah ﷻ knows of.
- At-Tadhkira (767). Al-Qurtubi said that the meaning of the phrase "the believers, after being saved from Hellfire" (in the hadith above in Sahih al-Bukhari 81/124) is referring to after being saved from falling off as-Sirāt into Hellfire, and that those are the believers that Allah ﷻ knows that qissās does not wear off their good deeds in entirety.
- Sharh al-'Aqīda al-Wāssitiyyah by Ibn Taymiyyah (3/163). Muhammad ibn al Uthaymeen said that the qissās that takes place on al-Qantara is different from the first qissās. The second one is aimed at removing any resentments that the dwellers of Paradise may have for each other. It is a purification process, rather than a retaliation one.
- Majmū' al-Fatāwa (15/344-345). Ibn Taymiyyah said that impure souls are not fit to dwell in Paradise where impurity is not an element there. The purification of such souls is a must as the phrase "after they are cleansed and purified through retaliation" (in the hadith above in Sahih al-Bukhari 81/124). This is what the Qur'an means when it says that all resentments within them will be removed (see Qur'an 15:47).