Tirmidhi

Explanation:
1:
This is a reference to the saying of Allah Ta'ala: ﴿مِنَ الْمُؤْمِنِينَ رِجَالٌ صَدَقُوا مَا عَاهَدُوا اللَّهَ عَلَيْهِ فَمِنْهُم مَّن قَضَى نَحْبَهُ وَمِنْهُم مَّن يَنتَظِرُ﴾ (, Al-Ahzab: 23) (meaning: Among the believers are men who have been true to their covenant with Allah (of patience and steadfastness); among them is he who has fulfilled his vow, and among them is he who is still waiting.)

Commentary:
(1)
In this hadith, there is an allusion to this blessed verse: ﴿مِنَ المُؤمِنينَ رِ‌جالٌ صَدَقوا ما عـهَدُوا اللَّهَ عَلَيهِ فَمِنهُم مَن قَضى نَحبَهُ وَمِنهُم مَن يَنتَظِرُ‌ وَما بَدَّلوا تَبديلًا﴾ (, Al-Ahzab: 23)
Among the believers are men who have been true to their covenant with Allah. Some of them have fulfilled their vow, and some are still waiting, and they have not changed in the least.

(2)
In this, there is the honor of Talhah radi Allahu anhu, that he was declared as one who fulfilled his promise even before martyrdom. It is as if the outstanding deeds he has accomplished up to now are so numerous and so great that he can be considered to possess the lofty rank of the martyrs even before actual martyrdom.

(3)
In this, there is also a testimony to his being a sincere believer, and that the promise he made to Allah is a true promise, made with sincerity of heart.