Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1102
Jabir reported: We were in the company of the Messenger of Allah (may peace be upon him), and he sent me on an errand, and when I came back (I saw him) saying prayer on his ride and his face was not turned towards Qibla. I greeted him but he did not respond to me. As he completed the prayer, he said: Nothing prevented me from responding to your greeting but the fact that I was praying.