Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 341
Undesirability of Glancing in one Direction of the other during Prayer

1755. `Aishah (May Allah be pleased with her) said: I asked the Messenger of Allah (PBUH) about random looks in Salat (prayer), and he replied, "It is something which Satan snatches from the slave's Salat.''
[Al-Bukhari].

Commentary:  "Pouncing'' or "snatching'' means to take away something swiftly from someone while he is not alert. When a person looks here and there in the course of Salat and does not concentrate on it, Satan avails this opportunity and spoils his Salat.

1756.  Anas (May Allah be pleased with him) said: The Messenger of Allah (PBUH) said to me, "Beware of looking around in Salat (prayer), because random looks in Salat are a cause of destruction. If there should be no help from it, it is permissible in the voluntary and not in obligatory Salat.''
[At-Tirmidhi].

Commentary:  In the chain of transmitters [i.e., Sanad] of this Hadith, we find `Ali bin Zaid bin Jad`an who is known to be Da`if (i.e., weak). If this narration is not reliable, then seeing here and there even in voluntary Salat is not permissible. However, if at all looking is inevitable, one can slightly turn his face because if one turns the whole body, his Salat would become invalid as he would not be facing the Qiblah which is essential for Salat.