(a) Amir bin Sa'd narrates from his father, Sayyiduna Sa'd (may Allah be pleased with him), that he said: I saw the Messenger of Allah (peace and blessings be upon him) giving two salams in prayer. When he gave the salam to the right he would say: 'As-salamu alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah). Then when he turned to the left he would say: 'As-salamu alaykum wa rahmatullah' (Peace be upon you and the mercy of Allah), until the whiteness of his cheeks could be seen from both sides.
(b) Isma'il bin Muhammad says: I mentioned this hadith before al-Zuhri, and al-Zuhri said: I have not heard this hadith among the hadiths of the Messenger of Allah (peace and blessings be upon him). Isma'il bin Muhammad said: Have you heard all the hadiths of the Messenger of Allah (peace and blessings be upon him)? Al-Zuhri said: No. He said: Two-thirds? He said: No. He said: Then half, at least? Al-Zuhri stopped at half or at a third. Then Isma'il said to him: Place this hadith among those you have not heard.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2980