It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to give the salam to his right and to his left, until his blessed cheek could be clearly seen, and then he would say: "'As-salamu alaykum wa rahmatullah' (Peace and the mercy of Allah be upon you)."
This hadith is established from Sayyiduna Sa'd bin Abi Waqqas and Sayyiduna Jabir bin Samurah (may Allah be pleased with them both).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2978