Musnad Ahmad 30:3754

Muhammad ibn Yusuf—meaning al-Firyabi—narrated to us in Makkah, al-Awza'i narrated to us, from Qurrah ibn Abd al-Rahman, from al-Zuhri, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Abbreviating the greeting is a Sunnah."