Musnad Abu Ya'la al-Mawsili 115:119

Shabab ibn Khayyat narrated to us, Durust ibn Hamzah narrated to us, Matar al-Warraq narrated to us, from Qatadah, from Anas, from the Prophet (peace and blessings of Allah be upon him), who said: "There are no two servants who love each other for the sake of Allah, such that one of them meets his companion and shakes his hand, and they send blessings upon the Prophet (peace and blessings of Allah be upon him), except that they do not separate until their sins are forgiven, both what preceded of them and what came after."