Sahih Ibn Khuzaymah

And in this there is also evidence that when the Imam turns his head to the right for Salam (he will greet the followers on his right side), and when he turns his head to the left for Salam, he will greet the followers on his left side.

Sayyiduna Sa’d bin Abi Waqqas (may Allah be pleased with him) narrates that I saw the Noble Prophet (peace and blessings be upon him) turning his face to the right and to the left while saying salam, until the whiteness of his blessed cheek would become visible. So, Imam Zuhri (may Allah have mercy on him) says that this has not been heard in the form of a hadith from the Messenger of Allah (peace and blessings be upon him). Then Isma’il bin Muhammad says, “Have you heard all the ahadith of the Noble Prophet (peace and blessings be upon him)?” He replied, “No.” He was asked, “Have you heard two-thirds?” He replied, “No.” He was asked, “Have you heard half?” He replied, “No.” Upon this, Isma’il said, “Then this hadith is from that half which you have not heard.”