Zakat (Kitab Al-Zakat)
Dawud :: Book 9 : Hadith 1668
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.