Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 66
It is narrated on the authority of Abu Musa Ash'ari: I asked the Messenger of Allah which (attribute) of Islam is more excellent. Upon this he remarked: One in which the Muslims are safe, protected from the tongue and hand of (other Muslims). Ibrahim b. Sa'id al-Jauhari has narrated this hadith with the same words in addition to these. The Messenger of Allah (may peace and blessings be upon him) was asked as to who amongst the Muslims is better, and the rest of the hadith was narrated like this.