The Rights of a Believer on a Fellow Believer.
Kitāb al-Muʾmin|Volume 1|Book 1|Chapter 4
97 - وعن أبي عبد الله (ع) قال ما عُبدَ اللهُ بشئ أفضلَ من أداء حقّ المؤمنِ.
97. It has been reported that Abu 'Abd Allah عليه السلام said: Allah has never been worshipped with anything better than fulfilling the right of a believer.