Mujahid (may Allah have mercy on him) narrates from Abu Ma'mar that the imam of the people of Makkah gave the salam on both sides. I mentioned it to Sayyiduna Abdullah (may Allah be pleased with him), and he (may Allah be pleased with him) said: 'Anna 'aliqaha?' (Where did he get this from?)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2972