The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6357
Abu Dharr reported Allah's Messenger (may peace be upon him) as saying: Abu Dharr, when you prepare the broth, add water to that and give that (as a present) to your neighbour.