The directive to spit to one’s left or beneath one’s foot applies only when one is outside the mosque; inside the mosque he should spit only into his garment.

Explanation
Al-Nawawī (d. 676 AH) cites this ḥadīth in Riyāḍ al-Ṣāliḥīn under the chapter “Anger When Allah’s Law Is Violated.” It relates that the Messenger ﷺ noticed a spot of phlegm on the qiblah wall of the mosque, whereupon he became angry, scraped it off with his hand, and said, “When any one of you stands for prayer, he is conversing with his Lord.” That is, in the course of the prayer he addresses Allah, and Allah answers him.

It is authentically reported that when the servant says, “All praise is due to Allah, Lord of the worlds,” Allah replies, “My servant has praised Me.” When he says, “The All-Merciful, the Most Merciful,” Allah says, “My servant has extolled Me.” When he says, “Master of the Day of Judgment,” Allah says, “My servant has glorified Me.” When he says, “You alone we worship and from You alone we seek help,” Allah says, “This is between Me and My servant, and for My servant is what he asks.” And when he says, “Guide us to the straight path,” Allah says, “This is for My servant, and for My servant is what he requested.”

Thus you are privately addressing Allah with His own words, calling upon Him, glorifying, praising, and exalting Him. He is before you—between you and the qiblah—though He is above the heavens, over His Throne. Yet He is still before you, for He encompasses all things, and “There is nothing like unto Him, and He is the All-Hearing, the All-Seeing” (Q 42:11).