Musnad Ahmad 962:180

Aswad ibn Amir narrated to us, he said: Al-Hasan ibn Salih narrated to us, from Abu al-Muhallab, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever begins with the greeting (salam) is the closest of people to Allah and His Messenger."