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: 6359
Abu Dharr reported: Allah's Apostle (may peace be upon him) said to me: Don't consider anything insignificant out of good things even if it is that you meet your brother with a cheerful countenance.