Muhammad bin 'Abdullah al-Ansari محمد بن عبد الله بن المثني - الأنصاري · The Book on Traveling

"The Messenger of Allah said to me: 'O my son! Beware of looking around during the Salat, for indeed looking around during Salat is destruction. If you must do so, then in the voluntary (prayers), not in the obligatory (prayers).'" (Da'if)