Imam ibnul Qayyim [rahimahullaah] stated:
Muhammad’s (sallal-laahu-alayhi-wasallam) Ummah is not in need of anyone after him; rather the only need they have is someone to convey what he came with. Muhammad’s (sallal-laahu-alayhi-wasallam) Message is comprehensive, universal and preserved. This universality, comprehensiveness and preservation are not limited to (any time and place). It is comprehensive and universal for those to whom Muhammad (sallal-laahu-alayhi-wasallam) was sent (i.e. the whole world) and it is likewise with regards to everything that is needed in the fundamental and subsidiary issues of the religion. Muhammad’s (sallal-laahu-alayhi-wasallam) Message is sufficient, all embracing and universal- nothing else is needed besides it. It has not isolated any affair of truth needed by his Ummah in their sciences and deeds.
The Messenger of Allaah (sallal-laahu-alayhi-wasallam) did not leave this world until he acquainted his Ummah with everything. He even taught them the etiquettes of using the toilet; the etiquettes of sexual relations; the etiquettes of sleep and waking up; the etiquettes of eating and drinking and the affairs related travelling. He acquainted them with the etiquettes of speech and silence; the etiquettes of keeping company with the people and isolation. He acquainted them with the affairs of illness, good health, poverty and wealth. He acquainted them with all the affairs of life and death.
He described Allaah’s throne, the angels, the jinn, the hell fire and the day of judgement, until as if you have seen all these things with the naked eye. He (sallal-laahu-alayhi-wasallam) informed them about their Lord and His Perfect Names and Attributes and that Allaah alone is their only true object of worship.
He acquainted them with the Prophets of old and their nations- the events that took place between them and their nations- until as if you were present at that time. He acquainted them with all the means to good and the paths of evil- in general and in detail- which no other prophet informed his nation. He acquainted them with death and what will take place in the grave- either enjoyment or punishment for the body and the soul.
He acquainted them with the evidences and proofs of Tawheed (i.e. the oneness of Allaah in His Lordship, Names and Attributes, and that Allaah alone has the right to be worshipped).
He acquainted them with Prophet hood and the affairs of the Afterlife, and refuted all the sects of disbelief and misguidance. He acquainted them with war strategies- how to confront the enemy; the way to achieve victory and conquests. If only they knew and understood these affairs in the correct manner, they will never be overcome by their enemy.
He acquainted them with the plots of Iblees and the ways through which he approaches them, and the ways to repel those plots and plans. He acquainted them with the soul- its characteristics and faults- in such a manner that they are not in need anyone else to tell them about it.
He taught them how to live their daily lives and had they acknowledged and acted upon this, their worldly affairs will be firmly established. He brought them all the means of success in this life and the next, and they do not need anyone besides him.
So how can it be said that the path of Muhammad (sallal-laahu-alayhi-wasallam) is in need of something else outside of it. The reason for such a claim is due to the weak and little understanding of a person. Allaah (The Most High) said:
أَوَلَمْ يَكْفِهِمْ أَنَّا أَنزَلْنَا عَلَيْكَ الْكِتَابَ يُتْلَىٰ عَلَيْهِمْ ۚ إِنَّ فِي ذَٰلِكَ لَرَحْمَةً وَذِكْرَىٰ لِقَوْمٍ يُؤْمِنُونَ
Is it not sufficient for them that We have sent down to you the Book (the Qur’an) which is recited to them? Verily, herein is mercy and a reminder (or an admonition) for a people who believe. [29:51]
Allaah (The Most High) said:
وَنَزَّلْنَا عَلَيْكَ الْكِتَابَ تِبْيَانًا لِّكُلِّ شَيْءٍ وَهُدًى وَرَحْمَةً وَبُشْرَىٰ لِلْمُسْلِمِينَ
And We have sent down to you the Book (the Qur’an) as an exposition of everything, a guidance, a mercy, and glad tidings for those who have submitted themselves (to Allah as Muslims). [16:89]
Allaah (The Most High) said:
يَا أَيُّهَا النَّاسُ قَدْ جَاءَتْكُم مَّوْعِظَةٌ مِّن رَّبِّكُمْ وَشِفَاءٌ لِّمَا فِي الصُّدُورِ وَهُدًى وَرَحْمَةٌ لِّلْمُؤْمِنِينَ
O mankind! There has come to you a good advice from your Lord (i.e. the Qur’an, ordering all that is good and forbidding all that is evil), and a healing for that (disease of ignorance, doubt, hypocrisy and differences, etc.) in your breasts, – a guidance and a mercy (explaining lawful and unlawful things, etc.) for the believers. [10:57]
[Source: I’laam Muwaqqi’een- 4/375-377- Abridged and slightly paraphrased]
Abu Mu’aawiyah (Abdullah Al-Gambi)