Musnad Abu Ya'la al-Mawsili 140:54

Ibrahim ibn Muhammad ibn ‘Ar‘arah narrated to us, Yusuf ibn Ya‘qub al-Sadusi narrated to us, Maymun ibn ‘Ajlan narrated to us, from Maymun ibn Siyah, from Anas ibn Malik, from the Prophet (peace and blessings of Allah be upon him), who said: "There are no two Muslims who meet and one of them takes the hand of his companion, except that it is a duty upon Allah to answer their supplication and not to turn their hands away until He forgives them."