Intro
This is a hadith that Ibn Kathir has given in terms of 3:81.  Ibn Kathir translated 3:81 as follows:

(81. And (remember) when Allah took the covenant of the Prophets, saying: “Take whatever I gave you from the Book and Hikmah, and afterwards there will come to you a Messenger confirming what is with you; you must, then, believe in him and help him.” Allah said: “Do you agree (to it) and will you take up Isri” They said: “We agree.” He said: “Then bear witness; and I am with you among the witnesses.”) 

Commentary on 3:81 by Ibn Kathir

Allah states that He took a pledge from every Prophet whom He sent from Adam until `Isa, that when Allah gives them the Book and the Hikmah, thus acquiring whatever high grades they deserve, then a Messenger came afterwards, they would believe in and support him. Even though Allah has given the Prophets the knowledge and the prophethood, this fact should not make them refrain from following and supporting the Prophet who comes after them. This is why Allah, the Most High, Most Honored, said

﴿وَإِذْ أَخَذَ اللَّهُ مِيثَـقَ النَّبِيِّيْنَ لَمَآ ءَاتَيْتُكُم مِّن كِتَـبٍ وَحِكْمَةٍ﴾

(And (remember) when Allah took the covenant of the Prophets, saying: “Take whatever I gave you from the Book and Hikmah.”) meaning, if I give you the Book and the Hikmah,

﴿ثُمَّ جَآءَكُمْ رَسُولٌ مُّصَدِّقٌ لِّمَا مَعَكُمْ لَتُؤْمِنُنَّ بِهِ وَلَتَنصُرُنَّهُ قَالَ ءَأَقْرَرْتُمْ وَأَخَذْتُمْ عَلَى ذلِكُمْ إِصْرِى﴾

(“and afterwards there will come to you a Messenger confirming what is with you; you must, then, believe in him and help him.” Allah said, “Do you agree (to it) and will you take up Isri”)

Ibn `Abbas, Mujahid, Ar-Rabi`, Qatadah and As-Suddi said that `Isri’ means, “My covenant.” Muhammad bin Ishaq said that,

﴿إِصْرِى﴾

(Isri) means, “The responsibility of My covenant that you took,” meaning, the ratified pledge that you gave Me.

﴿قَالُواْ أَقْرَرْنَا قَالَ فَاشْهَدُواْ وَأَنَاْ مَعَكُمْ مِّنَ الشَّـهِدِينَفَمَنْ تَوَلَّى بَعْدَ ذَلِكَ﴾

(They said: “We agree.” He said: “Then bear witness; and I am with you among the witnesses.” then whoever turns away after this,”) from fulfilling this pledge and covenant, c

﴿فَأُوْلَـئِكَ هُمُ الْفَـسِقُونَ﴾

(they are the rebellious.) `Ali bin Abi Talib and his cousin `Abdullah bin `Abbas said, “Allah never sent a Prophet but after taking his pledge that if Muhammad were sent in his lifetime, he would believe in and support him.” Allah commanded each Prophet to take a pledge from his nation that if Muhammad were sent in their time, they would believe in and support him. Tawus, Al-Hasan Al-Basri and Qatadah said, “Allah took the pledge from the Prophets that they would believe in each other”, and this statement does not contradict what `Ali and Ibn `Abbas stated.

Therefore, Muhammad is the Final Prophet until the Day of Resurrection. He is the greatest Imam, who if he existed in any time period, deserves to be obeyed, rather than all other Prophets. This is why Muhammad led the Prophets in prayer during the night of Isra’ when they gathered in Bayt Al-Maqdis (Jerusalem). He is the intercessor on the Day of Gathering, when the Lord comes to judge between His servants. This is Al-Maqam Al-Mahmud (the praised station) ﴿refer to 17:79﴾ that only Muhammad deserves, a responsibility which the mighty Prophets and Messengers will decline to assume. However, Muhammad will carry the task of intercession, may Allah’s peace and blessings be on him.

﴿أَفَغَيْرَ دِينِ اللَّهِ يَبْغُونَ وَلَهُ أَسْلَمَ مَن فِى السَّمَـوَتِ وَالاٌّرْضِ طَوْعًا وَكَرْهًا وَإِلَيْهِ يُرْجَعُونَ – قُلْ ءَامَنَّا بِاللَّهِ وَمَآ أُنزِلَ عَلَيْنَا وَمَآ أُنزِلَ عَلَى إِبْرَهِيمَ وَإِسْمَـعِيلَ وَإِسْحَـقَ وَيَعْقُوبَ وَالاٌّسْبَاطِ وَمَا أُوتِىَ مُوسَى وَعِيسَى وَالنَّبِيُّونَ مِن رَّبِّهِمْ لاَ نُفَرِّقُ بَيْنَ أَحَدٍ مِّنْهُمْ وَنَحْنُ لَهُ مُسْلِمُونَ – وَمَن يَبْتَغِ غَيْرَ الإِسْلَـمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِى الاٌّخِرَةِ مِنَ الْخَـسِرِينَ ﴾

The online version of Tafsir ibn Kathir doesn’t have the quote
It should be noted that Ahmadi’s refuse to give the full page quote by Ibn Kathir, they refuse to give a context to this statement of his, which is a hadith.

Commentary given by Ibn Kathir
By giving this hadith, Ibn Kathir is proving that all prophet’s that came before Muhammad (Saw) would be forced to follow him if they were on Earth.  That is all.