Narrated Abdullah ibn Umar: The Prophet (peace be upon him) 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.
Sunan Abi Dawud, Book 9, Hadith 117