Al-Mundhiri recorded the following chapter in his book Al-Targhib wal-Tarhib:

الترغيب في دعاء المرء لأخيه بظهر الغيب سيما المسافر

Encouragement for one to supplicate for his brother behind his back, especially the traveler.

Source: al-Targhīb wal-Tarhīb 4/83

Abu Darda reported: The Messenger of Allah, peace and blessings be upon him, said:

مَا مِنْ عَبْدٍ مُسْلِمٍ يَدْعُو لأَخِيهِ بِظَهْرِ الْغَيْبِ إِلاَّ قَالَ الْمَلَكُ وَلَكَ بِمِثْلٍ

No Muslim servant supplicates for his brother behind his back but that the angel says: And for you the same.

Source: Ṣaḥīḥ Muslim 2732, Grade: Sahih

Ibn al-Malak commented on this tradition, saying:

وهذا في الحقيقة دعاءٌ مِن المَلَك بِمثْلِ ما دعاه لأخيه قيل كان السَّلَفُ إذا أراد أن يدعوَ لنفسه يدعو لأخيه المسلم بتلك الدعوة ليدعوَ له المَلَكُ بِمثْلِها فيكون أعونَ للاستجابة

In reality, this is a supplication from the angel the same as he supplicated for his brother. It is said that the righteous predecessors, if they wanted to supplicate for themselves, they would supplicate for their Muslim brother. By this supplication, the angels would supplicate for them in the same way and it would be more likely to receive an answer.

Source: Sharḥ Maṣābīḥ al-Sunnah 3/71

Al-Ghazali comments on this tradition, saying:

من آدَابِ الْأُلْفَةِ وَالْأُخُوَّةِ الدعاء للأخ في حياته وبعد مماته بِكُلِّ مَا يُحِبُّهُ لِنَفْسِهِ وَلِأَهْلِهِ وَكُلِّ مُتَعَلِّقٍ به فتدعو له كما تدعو لنفسك ولا تفرق بين نفسك وبينه فإن دعاءك له دعاء لنفسك على التحقيق

Among the manners of fellowship is to supplicate on behalf of one’s brother, in his life and after his death, for everything he likes for himself and his household and everything concerning him. He should supplicate for him as he would for himself, there being no difference between them. Indeed, your supplication for him is a supplication for you, in reality.

Source: Iḥyā’ ‘Ulūm al-Dīn 2/186

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

ثَلَاثُ دَعَوَاتٍ مُسْتَجَابَاتٌ لَا شَكَّ فِيهِنَّ دَعْوَةُ الْمَظْلُومِ وَدَعْوَةُ الْمُسَافِرِ وَدَعْوَةُ الْوَالِدِ عَلَى وَلَدِهِ

Three supplications are answered without a doubt: the supplication of an oppressed person, the supplication of a traveler, and the supplication of a parent over his child.

Source: Sunan al-Tirmidhī 1905, Grade: Sahih

‘Uqbah ibn ‘Amir reported: The Messenger of Allah, peace and blessings be upon him, said:

ثَلَاثٌ مُسْتَجَابٌ لَهُمْ دَعْوَتُهُمْ الْمُسَافِرُ وَالْوَالِدُ وَالْمَظْلُومُ

The supplications of three people are answered: the traveler, the parents, and the oppressed.

Source: Musnad Aḥmad 27900, Grade: Jayyid