Musnad Ahmad 32:511

Muhammad ibn Bakr narrated to us, Maymun al-Mar'i narrated to us, Maymun ibn Siyah narrated to us, from Anas ibn Malik, from the Prophet of Allah, may Allah bless him and grant him peace, who said: "There are no two Muslims who meet and one of them takes the hand of his companion, except that it is a right upon Allah to be present for their supplication, and He does not separate their hands until He forgives them."