Praise be to Allah.
The sutrah is a confirmed Sunnah (Sunnah mu’akkadah). The Prophet (peace and blessings of Allaah be upon him) said: “When one of you prays, let him pray facing a sutrah, and let him draw close to it.” Narrated by Abu Dawood with a jayyid isnaad. When the Prophet (peace and blessings of Allaah be upon him) travelled, he would take with him a short piece of wood and pray facing it. It is a confirmed Sunnah but it is not obligatory, because it is proven that on some occasions, he (peace and blessings of Allaah be upon him) prayed without a sutrah.
With regard to that which invalidates the prayer, it is a donkey, a black dog, and an adult woman, because the Prophet (peace and blessings of Allaah be upon him) said: “The prayer of a Muslim man is invalidated if he does not have in front of him something like the back of a saddle, by (the passage in front of him of) a woman, a donkey or a black dog.” Narrated by Muslim in his Saheeh from the hadeeth of Abu Dharr (may Allaah be pleased with him). Muslim also narrated it from Abu Hurayrah (may Allaah be pleased with him), without mentioning the word black. The basic principle is that a general statement is to be interpreted in terms of the specific statement. In the hadeeth of Ibn ‘Abbaas it says “A woman who menstruates” i.e., an adult woman. The correct view is that which is indicated by the hadeeth: that these three invalidate the prayer.
With regard to the words of ‘Aa’ishah (may Allaah be pleased with her), they are her own opinion and conclusion. She said: “What a bad thing to liken us to donkeys and dogs.” She said that she used to lie in front of the Messenger of Allaah (peace and blessings of Allaah be upon him) when he was praying, but that is not passing in front, because lying is not called passing . The Sunnah concerning that was not known to her, and the one who learns something is a proof against the one who is unaware of it. If a person prays facing someone who is opposite him, sitting or lying down, that does not affect him. Rather what invalidates the prayer is if it passes in front of him from one side to another, if the one who passes is front is one of the three mentioned and passes in front of him, or between him and the sutrah. If the female is young and has not reached the age of puberty, or the dog is not black, or something such as a camel or sheep and the like passes in front, then none of these invalidate the prayer. But it is prescribed for the one who is praying not to let anything pass in front of him, even if it does not invalidate the prayer, because of the hadeeth of Abu Sa’eed al-Khudri (may Allaah be pleased with him), which says that the Prophet (peace and blessings of Allaah be upon him) said: “If one of you prays facing something that is screening him from the people, and someone wants to pass [directly] in front of him, let him push him away, and if he insists then let him fight him, for he is a devil.” Saheeh – agreed upon. End quote.