Tafsir Ibn Kathir -> Surah Fussilat -> The Call to Tawhid Allah says,

﴿قُلْ﴾

(Say) `O Muhammad, to these disbelievers and idolators,'

﴿إِنَّمَآ أَنَاْ بَشَرٌ مِّثْلُكُمْ يُوحَى إِلَىَّ أَنَّمَآ إِلَـهُكُمْ إِلَـهٌ وَاحِدٌ﴾

(I am only a human being like you. It is revealed to me that your God is One God,) `not like these idols and false gods which you worship. Allah is one God,'

﴿فَاسْتَقِيمُواْ إِلَيْهِ﴾

(therefore take straight path to Him) means, `worship Him Alone sincerely, in accordance with what He has commanded you through His Messengers.'

﴿وَاسْتَغْفِرُوهُ﴾

(and seek forgiveness of Him) means, `for your past sins.'

﴿وَوَيْلٌ لِّلْمُشْرِكِينَ﴾

(And woe to the polytheists.) means, doom and destruction is their lot.

﴿الَّذِينَ لاَ يُؤْتُونَ الزَّكَوةَ﴾

(Those who give not the Zakah) `Ali bin Abi Talhah reported from Ibn `Abbas that this means those who do not bear witness that there is no God except Allah. This was also the view of `Ikrimah. This is like the Ayat:

﴿قَدْ أَفْلَحَ مَن زَكَّـهَا - وَقَدْ خَابَ مَن دَسَّـهَا ﴾

(Indeed he succeeds who purifies himself. And indeed he fails who corrupts himself.) (91:9-10) And;

﴿قَدْ أَفْلَحَ مَن تَزَكَّى - وَذَكَرَ اسْمَ رَبِّهِ فَصَلَّى ﴾

(Indeed whosoever purifies himself shall achieve success. And remembers (glorifies) the Name of his Lord, and prays.) (87:14-15) And;

﴿فَقُلْ هَل لَّكَ إِلَى أَن تَزَكَّى ﴾

(And say to him: "Would you purify yourself'') (79:18) What is meant by Zakah here is purification of the soul, ridding oneself of all bad qualities, the worst of which is Shirk. The Zakah paid on one's wealth is so called because it purifies wealth, and it is a means of increasing it, blessing it and making it more beneficial, and a means of assisting one to use it in doing good deeds. Qatadah said, "They withheld the Zakah of their wealth.'' This is the apparent meaning according to many of the scholars of Tafsir, and this is the view favored by Ibn Jarir. But the matter is subject to further examination, because the obligation of Zakah was instituted during the second year after the Hijrah to Al-Madinah, according to what is stated by several scholars. Yet this Ayah was revealed in Makkah. However, it is not unlikely that the principle of giving charity and Zakah was already in place and had been enjoined at the beginning of the Prophet's mission, as Allah says:

﴿وَءَاتُواْ حَقَّهُ يَوْمَ حَصَادِهِ﴾

(but pay the due thereof on the day of their harvest) (6:141). As for the details of Zakah and how it is to be calculated according to the Nusub, were explained in Al-Madinah. This is how we may reconcile between the two opinions. Similarly, prayer was originally enjoined before sunrise and before sunset at the beginning of the Prophet's mission; it was only on the Night of the Isra', a year and a half before the Hijrah, that Allah enjoined upon His Messenger the five daily prayers. The conditions and essential elements of prayer were explained later, in stages. And Allah knows best. Then Allah says:

﴿إِنَّ الَّذِينَ ءامَنُواْ وَعَمِلُواْ الصَّـلِحَـتِ لَهُمْ أَجْرٌ غَيْرُ مَمْنُونٍ ﴾

(Truly, those who believe and do righteous good deeds, for them will be an endless reward that will never stop.) Mujahid and others said, "It will never be cut off or decrease.'' This is like the Ayat:

﴿مَّاكِثِينَ فِيهِ أَبَدًا ﴾

(They shall abide therein for ever.) (18:3)

﴿عَطَآءً غَيْرَ مَجْذُوذٍ﴾

(a gift without an end) (11:108)

﴿قُلْ أَءِنَّكُمْ لَتَكْفُرُونَ بِالَّذِى خَلَقَ الاٌّرْضَ فِى يَوْمَيْنِ وَتَجْعَلُونَ لَهُ أَندَاداً ذَلِكَ رَبُّ الْعَـلَمِينَ - وَجَعَلَ فِيهَا رَوَاسِىَ مِن فَوْقِهَا وَبَـرَكَ فِيهَا وَقَدَّرَ فِيهَآ أَقْوَتَهَا فِى أَرْبَعَةِ أَيَّامٍ سَوَآءً لِّلسَّآئِلِينَ - ثُمَّ اسْتَوَى إِلَى السَّمَآءِ وَهِىَ دُخَانٌ فَقَالَ لَهَا وَلِلاٌّرْضِ ائْتِيَا طَوْعاً أَوْ كَرْهاً قَالَتَآ أَتَيْنَا طَآئِعِينَ - فَقَضَاهُنَّ سَبْعَ سَمَـوَتٍ فِى يَوْمَيْنِ وَأَوْحَى فِى كُلِّ سَمَآءٍ أَمْرَهَا وَزَيَّنَّا السَّمَآءَ الدُّنْيَا بِمَصَـبِيحَ وَحِفْظاً ذَلِكَ تَقْدِيرُ الْعَزِيزِ الْعَلِيمِ ﴾

(9. Say: "Do you verily disbelieve in Him Who created the earth in two Days And you set up rivals with Him That is the Lord of all that exists.'') (10. He placed therein firm mountains from above it, and He blessed it, and measured therein its sustenance in four Days equal for all those who ask.) (11. Then He rose over (Istawa ila) the heaven when it was smoke, and said to it and to the earth: "Come both of you willingly or unwillingly.'' They both said: "We come willingly.'') (12. Then He completed and finished their creation (as) seven heavens in two Days and He made in each heaven its affair. And We adorned the nearest (lowest) heaven with lamps (stars) to be an adornment as well as to guard. Such is the decree of Him, the Almighty, the All-Knower.)