Musnad Ahmad 32:1278

'Abd al-Samad narrated to us, 'Abd Allah ibn al-Muthanna narrated to us, from Thumamah, from Anas: "That the Messenger of Allah (peace and blessings of Allah be upon him), when he spoke a word, he would repeat it three times. And when he came to a people and greeted them, he would greet them three times."