Mishkat al-Masabih 2260
Ibn ‘Abbas quoted the Prophet as saying, “Five supplications receive an answer:
that of one who is wronged till help comes, that of a pilgrim till he comes home, that of one engaged in jihad till he stops, that of a sick person till he recovers, and that of someone for an absent brother. Then he added, “The one of those which gets the quickest answer is that for an absent brother.” 

Baihaqi transmitted it in [Kitab] ad-Da'awat al-kabir .
وَعَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ 
" خَمْسُ دَعَوَاتٍ يُسْتَجَابُ لَهُنَّ: دَعْوَةُ الْمَظْلُومِ حَتَّى يَنْتَصِرَ وَدَعْوَةُ الْحَاجِّ حَتَّى يَصْدُرَ وَدَعْوَةُ الْمُجَاهِدِ حَتَّى يَقْعُدَ وَدَعْوَةُ الْمَرِيضِ حَتَّى يَبْرَأَ وَدَعْوَةُ الْأَخِ لِأَخِيهِ بِظَهْرِ الْغَيْبِ ". ثُمَّ قَالَ: «وَأَسْرَعُ هَذِهِ الدَّعْوَات إِجَابَة دَعْوَة الْأَخ لِأَخِيهِ بِظَهْرِ الْغَيْبِ» . رَوَاهُ الْبَيْهَقِيُّ فِي الدَّعَوَاتِ الْكَبِيرِ