Sahih Bukhari Volume 003, Book 031, Hadith Number 189.
Narated By Abu Juhaifa : The Prophet made a bond of brotherhood between Salman and Abu Ad-Darda.' Salman paid a visit to Abu Ad-Darda' and found Um Ad-Darda' dressed in shabby clothes and asked her why she was in that state. She replied, "Your brother Abu Ad-Darda' is not interested in (the luxuries of) this world." In the meantime Abu Ad-Darda' came and prepared a meal for Salman. Salman requested Abu Ad-Darda' to eat (with him), but Abu Ad-Darda' said, "I am fasting." Salman said, "I am not going to eat unless you eat." So, Abu Ad-Darda' ate(with Salman). When it was night and (a part of the night passed), Abu Ad-Darda' got up (to offer the night prayer), but Salman told him to sleep and Abu Ad-Darda' slept. After sometime Abu Ad-Darda' again got up but Salman told him to sleep. When it was the last hours of the night, Salman told him to get up then, and both of them offered the prayer. Salman told Abu Ad-Darda', "Your Lord has a right on you, your soul has a right on you, and your family has a right on you; so you should give the rights of all those who has a right on you." Abu Ad-Darda' came to the Prophet and narrated the whole story. The Prophet said, "Salman has spoken the truth."