It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that he said: the Messenger of Allah (peace and blessings be upon him) sent me on some errand. When I returned, he was praying on his mount facing the east, and he would bend lower in prostration than in bowing. I greeted him (peace and blessings be upon him) with salam, but he (peace and blessings be upon him) did not return my greeting. When he (peace and blessings be upon him) had finished the prayer, he (peace and blessings be upon him) said: "I was praying."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2211