Tirmidhi

Explanation:
1:
Because this is still from such spoils of war (mal-e-ghanimat) whose distribution has not yet taken place, so how can I give it to you?

2:
Because now, in the distribution (as khums), it has come into my share.

3:
These people ask you about the ruling concerning spoils of war (al-anfal: 1)

Hadith Commentary: Benefits and Issues: (1)
Anfal:
This is the plural of "nafl," which means surplus or addition. However, what is meant here? The scholars have the following differences regarding this:

(1)
By "anfal" is meant the spoils of war (ghanā’im), over which Allah gave the right of disposal to the Messenger. In this sense, this verse would be considered abrogated, because later, four shares of the spoils were allotted to the warriors, and the fifth share was left to the discretion of the Messenger sallallahu alayhi wa sallam. See Hadith number (4556).

(2)
By "anfal" is meant the "khums" (the fifth share), not the entire spoils of war. In this case, this verse would not be considered abrogated.

(3)
By "anfal" is meant "fay’," that is, the wealth which comes to the Muslims from the disbelievers without battle or fighting. In this, the Prophet could dispose as he wished.

(4)
By "anfal" is meant that gift or reward which the Imam grants to someone for good performance.

(5)
By "anfal" is meant that gift or reward which the Imam gives additionally to a detachment when he sends them on a special mission separate from the main army.

(2)
Sa‘d radi Allahu anhu took a sword from the spoils of war. It was referred to as "khums" because after the Battle of Badr, when there arose a dispute regarding the distribution of spoils and the rulings concerning it were revealed in the Noble Qur’an, whatever was given to a warrior as a gift or reward was counted from the "khums." Since the rulings had not yet been revealed at that time, and through the verse of Anfal, the authority was given to the Prophet sallallahu alayhi wa sallam, he sallallahu alayhi wa sallam granted that sword to Sa‘d radi Allahu anhu.

Hadith Commentary: Benefits and Issues: (1)
The four verses that were revealed concerning Hazrat Sa'd radi Allahu anhu, the Imam will mention them ahead in the Book of Virtues (Kitab al-Fada'il), namely: dutifulness and good conduct towards parents (birr al-walidayn), the prohibition of wine, the verse ﴿وَلَا تَطْرُدِ الَّذِينَ يَدْعُونَ رَبَّهُم﴾ ("And do not drive away those who call upon their Lord"), do not rebuff those who call upon their Lord, and the verse of al-Anfal.

(2)
Hazrat Sa'd radi Allahu anhu played a significant role in the Battle of Badr. He had killed Sa'id ibn al-As, one of the prominent warriors of the disbelievers of Quraysh. Therefore, he believed that he had a right to his sword. Furthermore, his brother Umair had also been killed, so he was greatly distressed and desirous of his (Sa'id's) acceptance of faith. For this reason, he insisted greatly on taking the sword.

Benefits and Issues:
In the well-known recitation, (يَسْأَلُونَكَ عَنِ الْأَنفَالِ) means:
People ask you about the ruling concerning spoils of war (ghanā’im).
And in the recitation of Ibn Mas‘ud radi Allahu anhu, (يَسْأَلُونَكَ النَّفْل) is translated as:
People ask you, O Prophet sallallahu alayhi wa sallam, about the voluntary (nafl) [reward].
(i.e., about an additional reward or bounty)