Musnad Ahmad 962:100

Khalaf ibn al-Walid narrated to us, Ibn al-Mubarak narrated to us, and Ali ibn Ishaq, Ibn al-Mubarak informed us, from Yahya ibn Ayyub, 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: "Part of the perfection of visiting the sick is that one of you places his hand upon his forehead, or his hand, and asks him how he is. And the perfection of your greetings among yourselves is the handshake."