The sutrah is obligatory for the imam and for anybody praying alone. The evidence for this is that the Prophet (May Allāh praise him and Peace be upon him) said, “Do not pray without a sutrah and do not allow anyway to pass in front of you. If he resists you, fight him because he has a companion (i.e. a devil) with him” [Ṣifat al Ṣalāt (Description of Prayer), p.82].

Sheikh Al Albānī (may Allāh have mercy on him) said, “One thing that confirms the obligation of using a sutrah for prayer is that it is a means that is prescribed in Islam for preventing one’s prayer from being invalidated by a woman who has come of age, a donkey or a black dog passing in front of him, as is authentically established in the ḥadīth (narration), and to prevent anybody from passing in front of him” [Tamām Al Minnah: p.300].