Good Manners and Form (Al-Adab)
Ṣaḥīḥ al-Bukhārī 6111
إِنَّ أَحَدَكُمْ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّ اللَّهَ حِيَالَ وَجْهِهِ، فَلاَ يَتَنَخَّمَنَّ حِيَالَ وَجْهِهِ فِي الصَّلاَةِ
inna aḥadakum idhā kāna fī aṣ-ṣalāhi fa-inna-Llāh ḥiyāla wajhihi, falā yatanakhamanna ḥiyāla wajhihi fī aṣ-ṣalāhi
Transliteration generated from the Arabic vowel marks
The full narration
Narrated `Abdullah bin `Umar:While the Prophet (ﷺ) was praying, he saw sputum (on the wall) of the mosque, in the direction of the Qibla, and so he scraped it off with his hand, and the sign of disgust (was apparent from his face) and then said, "Whenever anyone of you is in prayer, he should not spit in front of him (in prayer) because Allah is in front of him
Source
Ṣaḥīḥ al-Bukhārī · 6111
Ṣaḥīḥ