Hadith: I asked the Messenger of Allah (may Allah's peace and blessings be upon him) about looking around during prayer. He replied: It is a way of stealing by which the devil steals a portion of a slave's prayer

عن عائشة رضي الله عنها قالت: سَألتُ رسول الله صلى الله عليه وسلم عن الالتِفَات في الصلاة؟ فقال: «هو اخْتِلاس يَختَلِسُهُ الشَّيطان من صلاة العَبْد».
[صحيح] - [رواه البخاري]

‘Ā’ishah (may Allah be pleased with her) reported: I asked the Messenger of Allah (may Allah's peace and blessings be upon him) about looking around during prayer. He replied: "It is a way of stealing by which the devil steals a portion of a slave's prayer."
Sahih/Authentic. - [Al-Bukhari]

Explanation

‘Ā’ishah (may Allah be pleased with her) asked the Messenger of Allah (may Allah’s peace and blessings be upon him) about the ruling on looking hither and thither during the prayer, and whether it affects the validity of prayer. He told her that this looking is a means by which the devil snatches away a portion of a slave’s prayer in a swift and hidden manner so as to affect its validity and decrease its reward.