His prayer is invalidated; as narrated by Jabir, may Allah be pleased with him, who said: ((The Messenger of Allah, peace be upon him, sent me while he was heading to Banu al-Mustaliq, so I came to him while he was praying on his camel and spoke to him, and he gestured to me with his hand like this, then I spoke to him again, and he gestured to me like this, and I heard him reciting while nodding his head. When he finished, he said: What did you do regarding what I sent you for? The only thing that prevented me from speaking to you was that I was praying)), in Sahih Muslim 1: 383. See: Durar al-Hukkam 1: 102, and Fath Bab al-Inayah 1: 303.