Assalamualaikum ustaz. What is the ruling of a makmum who prostrate for too long in the last prostration when the imam has already performed the final tahiyyat? Hope for an explanation.
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
The word imam means someone who is followed by others, such as an imam in prayer, as a caliph, or general in the war.  The word imam itself gives the meaning of being in the forefront and a person who is referred to while his followers are known as the makmum.
Basically, the makmum is obligated to follow the imam in his every movement during congregational prayer. This is based on the hadith narrated by Abu Hurairah. The Prophet PBUH said:
إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ فَلَا تَخْتَلِفُوا عَلَيْهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا لَكَ الْحَمْدُ وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا أَجْمَعُونَ
“The Imam is to be followed. Say the Takbir when he says it; bow if he bows; if he says ‘Sami`a l-lahu liman hamidah’, say, ‘ Rabbana wa laka l-hamd’, prostrate if he prostrates and pray sitting altogether if he prays sitting.”
Imam al-Nawawi when commenting on this hadith said that a makmum is obligated to follow the imam during takbir, qiyam, bow (rukuk) and prostration (Sujud) and others. Makmum must perform takbir right after the imam. If a makmum performs any actions in prayer before the imam completes his, then his prayer is invalid. 
In another hadith, Anas RA said:
صَلَّى بِنَا رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ ذَاتَ يَوْمٍ فَلَمَّا قَضَى الصَّلَاةَ أَقْبَلَ عَلَيْنَا بِوَجْهِهِ، فَقَالَ: أَيُّهَا النَّاسُ، إِنِّي إِمَامُكُمْ، فَلَا تَسْبِقُونِي بِالرُّكُوعِ وَلَا بِالسُّجُودِ، وَلَا بِالْقِيَامِ وَلَا بِالِانْصِرَافِ، فَإِنِّي أَرَاكُمْ أَمَامِي وَمِنْ خَلْفِي
God’s Messenger led us in prayer one day, and when he finished his prayer he faced us and said, “O people, I am your imam, so do not bow, prostrate yourselves, stand, or go away before I do, for I see you both in front of me and behind me.” 
According to the above hadith, scholars concluded that the valid condition of congregational prayer is to follow every action of the imam. This matter is stated by Imam al-Nawawi: “It is obligatory to follow the imam in every action of prayer which means he moves immediately after the imam completes the imam’s action before he starts his.” 
In a hadith of Abu Hurairah RA, it is narrated that Rasullullah PBUH said:
إِنَّمَا جُعِلَ الْإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا كَبَّرَ فَكَبِّرُوا، وَإِذَا رَكَعَ فَارْكَعُوا، وَإِذَا رَفَعَ رَأْسَهُ فَارْفَعُوا رُءُوسَكُمْ، وَإِذَا قَالَ: سَمِعَ اللهُ لِمَنْ حَمِدَهُ فَقُولُوا جَمِيعًا: اللهُمَّ رَبَّنَا لَكَ الْحَمْدُ، وَإِذَا سَجَدَ فَاسْجُدُوا، وَلَا تَسْجُدُوا قَبْلَ أَنْ يَسْجُدَ، وَإِذَا رَفَعَ رَأْسَهُ فَارْفَعُوا رُءُوسَكُمْ، وَلَا تَرْفَعُوا رُءُوسَكُمْ قَبْلَ أَنْ يَرْفَعَ
“The Imam is appointed so that he should be followed, so don’t be at variance with him. Recite takbir when he recites it; bow down when he bows down and when he stands (for I’tidal) then you should stand. If he says: Sami`allahu liman hamidah, all of you should recite: Allahumma rabbana wa lakal hamdu. And when he (the Imam) prostrates, you should also prostrate, and only when he raises his head then you raise your head and don’t raise your head before the imam raises his head.” 
Hence, when a makmum is behind an imam, he should follow the actions of the imam according to the above-stated evidences.
In the above issue, the action is makruh (discouraged). The reason is a makmum should follow the imam in accordance with a long hadith of al-Musi’ fi al-Solat, the Messenger PBUH said:
إنما جُعِلَ الإمام ليُؤتمَّ به، فإذا كبَّر فكبِّروا، وإذا رَكَعَ فاركعوا
“The Imam is appointed to be followed. So, recite takbir when he recites it, and bow down when he bows down.” 
Imam al-Nawawi explained: “It means according to al-Syaife and a group of scholars (following) the apparent actions in prayer.” 
Here, we’ll also cite a paragraph from the book al-Fiqh al-Manhaji:
“A makmum should follow the changes in the actions of the imam in all fi’li requisites. The actions of the makmum must follow the actions of the imam before the imam completes the action. It is makruh for the makmum to delay or late in following the imam for one requisite and his prayer is invalidated if it is for two long requisites. For example, the imam has bowed and I’tidal, then prostrated and sit while the makmum is still standing without any forgivable reason.
However, if he is left due to forgivable reasons such as his recitation being slow, it is permissible for him to be late from the imam for three requisites. Although, if this time is still not enough for him to follow the imam, he should stop his actions and proceed to follow the imam. Afterwards, he has to perform his missed actions after the imam gives the salam.”