Musnad Abu Ya'la al-Mawsili 106:20

Khalid ibn Mirdas narrated to us, Hushaym narrated to us, from Abu Balj, from Zayd ibn Abi al-Sha'tha' al-'Abdi, from al-Bara', who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When two Muslims meet and shake hands, and praise Allah and seek His forgiveness, they are both forgiven."