Because Allah, the Mighty and Majestic, is towards the face of the one who is praying as long as the worshipper remains attentive towards Him in his prayer.

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings of Allah be upon him) saw phlegm in the direction of the qiblah in the mosque, so he (peace be upon him) rubbed it off with his hand or said that he scraped it off with his hand. Then he turned towards the people and expressed severe displeasure at them. And he said: "Indeed, Allah Almighty is before the face of any one of you in his prayer. Therefore, let no person ever, ever spit phlegm in the direction of his face during his prayer."

Sayyiduna Abu Wa'il رضي الله عنه narrates that when Shaith bin Rabi'i prayed standing next to Sayyiduna Hudhayfah رضي الله عنه, he spat in front of himself. Sayyiduna Hudhayfah رضي الله عنه said: Indeed, the Messenger of Allah صلى الله عليه وسلم forbade us from this and said, "When a person begins prayer, Allah Almighty turns towards him with His noble face and does not turn away from the worshipper until he himself becomes inattentive to Allah Almighty or his ablution breaks."

Sayyiduna Hudhayfah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever spits in the direction of the Qiblah will come on the Day of Resurrection in such a state that his spit will be between his two eyes."

Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) saw phlegm in the direction of the qiblah in the mosque, so he cleaned it with his stick. Then he turned towards the people, and they could see intense anger on his blessed face. He (peace and blessings be upon him) asked, "Whose phlegm is this?" They all remained silent. He (peace and blessings be upon him) said, "Does any one of you like that when he is praying, a person comes and spits on his face?" The companions replied, "No, (no one would like that)." He (peace and blessings be upon him) said, "Indeed, Allah is in front of you in your prayers, so do not throw any filth in front of yourselves, but (if necessary) put it to your left or under your foot."

Imam Abu Bakr, may Allah have mercy on him, says that I have previously dictated some ahadith in which these words are present.

Lady Aisha (may Allah be pleased with her) narrates that the Messenger of Allah (peace and blessings be upon him) prayed in a cloak with designs and patterns, then said: "Its decorations distracted me. Therefore, take this cloak to Abu Juham and bring me a (plain) cloak made in Anbijan." In the narration from Zuhri from Makhzumi, there are also the words «‏‏‏‏انبجانية» ‏‏‏‏. The Imam narrates this same narration through another chain as well.

The Imam narrates this same tradition through another chain of transmission.

Sayyiduna Harith Ash’ari (may Allah be pleased with him) narrates that the Prophet Muhammad (peace and blessings be upon him) informed the Companions (may Allah be pleased with them) that“Allah Almighty commanded Yahya bin Zakariya (peace be upon him) to act upon five things and to command the Children of Israel to act upon them as well. But he delayed in commanding them, so Isa (peace be upon him) said to him, ‘Indeed, you have been commanded to act upon five things and to command the Children of Israel to act upon them. Therefore, either you command them regarding these matters, or I will stand and command them regarding these matters.’ Yahya (peace be upon him) said, ‘If you convey these words to the people before me, I fear that I may be punished or that I may be swallowed up by the earth.’ So he gathered the Children of Israel in Bayt al-Maqdis until the mosque was filled, and people sat on high mounds. Then he admonished and advised the people, and then said, ‘Indeed, Allah Almighty has commanded me with five things: that I act upon them and that I command you to act upon them as well. The first of these is that you do not associate anything with Allah. For whoever associates anything with Allah, his example is like that of a slave whom a man bought with his best wealth—gold or silver—then said to him,“This is my house and my business, so now work for me and give me your earnings.”So the slave began to work, but he started giving his earnings to someone other than his master. Which one of you would like to have such a slave, who gives his earnings to someone other than his master? (Remember) Indeed, Allah Almighty created you and provided for you, so do not associate anything with Him.’ And he said: ‘Indeed, Allah, the Mighty and Majestic, commands you to pray. (Therefore, when you stand for prayer) straighten your faces and do not look here and there, for Allah also turns His noble face towards His servant when he prays for Him. Then He does not turn His blessed face away until the servant turns his face away.’ And then he narrated the complete hadith.”

And the explanation of the evidence that turning the face during prayer does not make it obligatory to repeat the prayer.