Sunan Abi Dawud 1672
Narrated Abdullah ibn Umar:
The Prophet (ﷺ) said: If anyone seeks protection in Allah's name, grant him protection; if anyone begs in Allah's name, give him something; if anyone gives you an invitation, accept it; and if anyone does you a kindness, recompense him; but if you have not the means to do so, pray for him until you feel that you have compensated him.
حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ مُجَاهِدٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ مَنِ اسْتَعَاذَ بِاللَّهِ فَأَعِيذُوهُ وَمَنْ سَأَلَ بِاللَّهِ فَأَعْطُوهُ وَمَنْ دَعَاكُمْ فَأَجِيبُوهُ وَمَنْ صَنَعَ إِلَيْكُمْ مَعْرُوفًا فَكَافِئُوهُ فَإِنْ لَمْ تَجِدُوا مَا تُكَافِئُونَهُ فَادْعُوا لَهُ حَتَّى تَرَوْا أَنَّكُمْ قَدْ كَافَأْتُمُوهُ ‏"‏ ‏.‏