What is the ruling on the prayer of one who prayed behind an imam who said the taslim once without turning his head?

Perhaps the Imam follows the view that one Tasleem is sufficient. However, the Sunnah is for the worshipper to say the Tasleem to his right and to his left upon concluding the prayer. In Sahih Muslim, from Amir bin Sa'd, from his father, he said: "I used to see the Messenger of Allah, peace and blessings be upon him, give the Tasleem to his right and to his left until I would see the whiteness of his cheek."