Musnad Ahmad 1013:67

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah narrated to us, from Muhammad ibn Abdullah ibn Malik, from Sahl ibn Sa'd al-Ansari, that the Messenger of Allah (peace and blessings of Allah be upon him) used to: "Deliver the taslim in his prayer to his right and to his left, until the whiteness of his cheeks could be seen."