The Reason behind revealing Ayah 8:1

Imam Ahmad recorded that Sa`d bin Malik said, "I said, `O Allah's Messenger, Allah has brought comfort to me today over the idolators, so grant me this sword.' He said,

« إِنَّ هَذَا السَّيْفَ لَا لَكَ وَلَا لِي، ضَعْه »

(This sword is neither yours nor mine; put it down.) So I put it down, but said to myself, `The Prophet might give this sword to another man who did not fight as fiercely as I did.' I heard a man calling me from behind and I said, `Has Allah revealed something in my case' The Prophet said,

« كُنْتَ سَأَلْتَنِي السَّيْفَ وَلَيْسَ هُوَ لِي، وَإِنَّهُ قَدْ وُهِبَ لِي، فَهُو لَك »

(You asked me to give you the sword, but it is not for me to decide about. However, it has been granted to me (by Allah), and I give it to you.) So Allah sent down this Ayah,

﴿ يَسۡـَٔلُونَكَ عَنِ ٱلۡأَنفَالِ‌ۖ قُلِ ٱلۡأَنفَالُ لِلَّهِ وَٱلرَّسُولِ‌ۖ ﴾

(They ask you about Al-Anfal. Say: "Al-Anfal are for Allah and the Messenger'').

Abu Dawud, At-Tirmidhi and An-Nasa'i collected this Hadith, At-Tirmidhi said, "Hasan Sahih''.