Abu Dawud

Benefits and Issues:
In the early days of Islam, the hypocrites would not go out with the Messenger of Allah (sallallahu alayhi wa sallam) for jihad. Even if they did go, they would return using various excuses and pretexts. This has been mentioned regarding them in Surah At-Tawbah:

﴿لَا يَسْتَأْذِنُكَ الَّذِينَ يُؤْمِنُونَ بِاللَّـهِ وَالْيَوْمِ الْآخِرِ أَن يُجَاهِدُوا بِأَمْوَالِهِمْ وَأَنفُسِهِمْ ۗ وَاللَّـهُ عَلِيمٌ بِالْمُتَّقِينَ ﴿٤٤﴾ إِنَّمَا يَسْتَأْذِنُكَ الَّذِينَ لَا يُؤْمِنُونَ بِاللَّـهِ وَالْيَوْمِ الْآخِرِ وَارْتَابَتْ قُلُوبُهُمْ فَهُمْ فِي رَيْبِهِمْ يَتَرَدَّدُونَ﴾ (At-Tawbah: 44-45)

Those who have believed in Allah and the Last Day do not ask your permission to be excused from striving with their wealth and their lives. And Allah is Knowing of the righteous. Only those ask your permission who do not believe in Allah and the Last Day and whose hearts have doubted, and so they, in their doubt, are hesitating.

Those who have believed in Allah and the Day of Judgment do not ask you (sallallahu alayhi wa sallam) for permission so that they may not have to strive with their wealth or their lives. And Allah knows well those who are righteous. Only those people ask you for permission who do not believe in Allah and the Last Day. There is doubt in their hearts, and they are wandering in their doubts.

After the revelation of these verses, returning from jihad became prohibited, even if it was with the permission of the Prophet (sallallahu alayhi wa sallam). However, when Islam and the Muslims gained strength, and the number of Muslims increased, then the concession of returning after seeking permission was granted. And this verse of Surah An-Nur was revealed:

﴿إِنَّمَا الْمُؤْمِنُونَ الَّذِينَ آمَنُوا بِاللَّـهِ وَرَسُولِهِ وَإِذَا كَانُوا مَعَهُ عَلَىٰ أَمْرٍ جَامِعٍ لَّمْ يَذْهَبُوا حَتَّىٰ يَسْتَأْذِنُوهُ ۚ إِنَّ الَّذِينَ يَسْتَأْذِنُونَكَ أُولَـٰئِكَ الَّذِينَ يُؤْمِنُونَ بِاللَّـهِ وَرَسُولِهِ ۚ فَإِذَا اسْتَأْذَنُوكَ لِبَعْضِ شَأْنِهِمْ فَأْذَن لِّمَن شِئْتَ مِنْهُمْ وَاسْتَغْفِرْ لَهُمُ اللَّـهَ ۚ إِنَّ اللَّـهَ غَفُورٌ رَّحِيمٌ) (An-Nur: 62)

The believers are only those who have believed in Allah and His Messenger (sallallahu alayhi wa sallam), and when they are with him in a collective matter, they do not leave until they have asked his permission. Indeed, those who ask your permission—those are the ones who believe in Allah and His Messenger (sallallahu alayhi wa sallam). So when they ask your permission for some affair of theirs, then give permission to whom you will among them and ask Allah's forgiveness for them. Indeed, Allah is Forgiving, Merciful.

The true believers are only those who have faith in Allah and His Messenger (sallallahu alayhi wa sallam). And when they are engaged with him in a collective matter, they do not depart until they have sought your permission. Surely, those who seek your permission—these are the ones who have believed in Allah and His Messenger (sallallahu alayhi wa sallam). So when they ask you for permission regarding some of their affairs, then grant permission to whomever you wish among them, and seek forgiveness for them from Allah. Surely, Allah is Most Forgiving, Most Merciful.