Hadith: When one of you stands to pray, he is privately conversing with his Lord, or his Lord is between him and the prayer direction. So let no one of you spit in the direction of prayer, but only to his left or under his feet

عن أنس بن مالك رضي الله عنه أن النبي صلى الله عليه وسلم رأى نُخْامَة في القِبْلَة، فَشَقَّ ذلك عليه حتى رُئِي في وجْهِه، فقام فَحَكَّه بِيَده، فقال: «إن أحدكم إذا قام في صلاته فإنه يُنَاجِي رَبَّه، أو إن رَبَّه بينه وبين القِبْلَة، فلا يَبْزُقَنَّ أحدُكم قِبَل قِبْلتِه، ولكن عن يَسَاره أو تحت قَدَمَيه» ثم أخذ طَرف رِدَائِه، فَبَصَقَ فيه ثم ردَّ بَعْضَهُ على بعض، فقال: «أو يفعل هكذا».
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) saw mucus in the prayer's direction. That was difficult for him to bear, and the feeling was even reflected in his face. Therefore, he stood up and scratched the mucus with his hand and said: "When one of you stands to pray, he is privately conversing with his Lord, or his Lord is between him and the prayer direction. So let no one of you spit in the direction of prayer, but only to his left or under his feet." Then he took a corner of his garment, spat into it, folded it up, and said: "Or he should do like this."
Authentic hadith - Narrated by Bukhari & Muslim

Explanation

The Prophet (may Allah's peace and blessings be upon him) saw mucus on the wall of the mosque toward the prayer direction. This was difficult for him to bear and he got noticeably upset. So he stood up and scraped the mucus off with his noble hand. In doing so, he was teaching his community to be humble toward Allah and to respect His house. The Prophet noted that when someone stands to pray, he communicates with Allah, mentions His name, and recites His revelation. So the appropriate thing to do is to be humble and to fully concentrate. Also, one has to remember the greatness of Allah and avoid inappropriate manners, including spitting in the direction of the prayer. If one needs to do so, he may spit in a handkerchief or so. In the prayer, one should feel that he is meeting Allah, the Most High, so one has to turn to Allah with complete devotion. Even though Allah is in the heavens above His throne, He is in front of the person praying, because He encompasses everything: {There is nothing like unto Him, and He is the All-Hearing, the All-Seeing} [Sūrat Ash-Shūra: 11]. This does not mean that Allah mixes with people or that He is in the place where one prays. Allah is close to those who pray to Him and those who supplicate to Him. His nearness befits His majesty; no human assimilation is at all contemplated. To make this point relevant, the sun is above us, but is also in front of us when it rises and when it sets.