Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) used to give the salam twice (that is, on both sides).
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Baqiyyah, who is thiqah (trustworthy) and a mudallis (one who practises tadlis), and he has narrated it as a mu'an'an narration (using 'from').
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2880