Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6274
'A'isha, the wife of Allah's Apostle (may peace be upon him), reported Allah's Apostle (may peace be upon him) as saying: Kindness is not to be found in anything but that it adds to its beauty and it is not withdrawn from anything but it makes it defective.