In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Allaah (The Most High) said:
إِذَا جَآءَ نَصۡرُ ٱللَّهِ وَٱلۡفَتۡحُ (١) وَرَأَيۡتَ ٱلنَّاسَ يَدۡخُلُونَ فِى دِينِ ٱللَّهِ أَفۡوَاجً۬ا (٢) فَسَبِّحۡ بِحَمۡدِ رَبِّكَ وَٱسۡتَغۡفِرۡهُۚ إِنَّهُ ۥ ڪَانَ تَوَّابَۢا (٣)
When comes the Help of Allah (to you, O Muhammad) against your enemies) and the conquest, And you see that the people enter Allah’s religion (Islam) in crowds, So glorify the Praises of your Lord, and ask for His Forgiveness. Verily, He is the One Who accepts the repentance and forgives.
This Surah conveys a great promise and a manifest victory to the Prophet (sallal-laahu-alayhi-wasallam).
Regarding the statement of Allaah [إِذَا جَآءَ نَصۡرُ ٱللَّهِ وَٱلۡفَتۡحُ – When comes the Help of Allah (to you, O Muhammad) and the conquest]- that is the conquest of Makkah, which shows the greatness of Allaah’s favour upon you [O Messenger (sallal laahu alayhi wasallam)] and that the conquest of Makkah is an event that will certainly take place.
And regarding Allaah’s statement:[فَسَبِّحۡ بِحَمۡدِ رَبِّكَ وَٱسۡتَغۡفِرۡهُۚ إِنَّهُ ۥ ڪَانَ تَوَّابَۢا – So glorify the Praises of your Lord, and ask for His Forgiveness. Verily, He is the One Who accepts the repentance and forgives]- that is, ‘’Increase in glorifying your Lord and seeking His forgiveness’’. After the revelation of this surah, the Messenger (alayhis-salaatu was-salaam) increased in saying: [سُبْحَانَكَ اللَّهُمَّ رَبَّنَا وَبِحَمْدِكَ اللَّهُمَّ اغْفِرْ لِي – I Glorify You (freeing You from all imperfections, faults, weakness, dificiencies); O Allaah! Our Lord! All Praises and Thanks are for You. O Allaah! Forgive me]. In this way he [the Prophet (sallal-laahu-alayhi-wasallam)] was acting according to the Qur’aan. [Saheeh al-Bukhaari. Hadeeth Number 817]
And from the benefits derived from this Surah:
The Prophet (sallal-laahu-alayhi-wasallam) realised that his lifespan would come to an end when this victory and conquest takes place. That is because the great acts of obedience to Allaah are finally sealed with Istighfaar and likewise a noble life of Imaan and obedience to Allaah is finally sealed with Istighfaar. So the last statement heard from the Prophet (alayhis-salaatuwas-salaam) before he passed away was: [اللَّهُمَّ اغْفِرْ لِي وَارْحَمْنِي وَأَلْحِقْنِي بِالرَّفِيقِ -O Allaah! Forgive me, and bestow Your Mercy on me, and let me meet the (highest) companions (of the Hereafter)] [Saheeh al-Bukhaari. Hadeeth Number 4440]
[Slightly paraphrased. Source: Sharh Ad-Duroos Al-Muhimmah Li-Aammatil Ummah page 34-35′ by Sh Abdur Razzaaq Al-Badr(may Allaah preserve him)]