Musnad Ahmad 33:230

Hashim narrated to us, Zuhayr narrated to us, Abu al-Zubayr narrated to us, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) sent me while he was setting out toward Banu al-Mustaliq. I came to him while he was praying upon his camel, and I spoke to him. He gestured with his hand like this. Then I spoke to him, and he gestured with his hand like this, while I heard him reciting and gesturing with his head. When he finished, he said: "What did you do regarding that which I sent you for? For nothing prevented me [from answering you] except that I was praying."