It is narrated from Sayyiduna Samurah bin Jundub (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to give one salam in prayer, facing forward, and when he gave the salam to his right, he would then give it to his left as well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2988