Foul language is a very general topic and does not just mean name-calling and swearing. There are other forms of foul language as well, for example, backbiting (talking ill of others) or even slandering (to wrongly accuse someone).
Foul language does not just hurt the person you directed your words to but also displeases God. Every time vulgar language is used, it slowly shapes the person you become. It makes you a less attractive person to be around and is a sin according to Islam. Islam teaches not to retaliate against someone who verbally abuses you, for example, if someone uses foul language against you, you should just reply calmly and with respect.
There are many Quranic verses and Ahadith on this topic. For example in Surah Furqan verse 63 the Holy Quran states; “And when the foolish address them (with bad language) they reply back with peaceful words of gentleness.”
In a Hadith, the Holy Prophet Muhammad (sa) said, “Backbiting is worse than killing.” Backbiting is talking ill of someone in their absence. The Promised Messiah (as) did not like people who abused others. For example Noah’s Ark outlines for us how we should act as Ahmadi Muslims. He writes “Be kind and merciful to humanity, for all are His creatures: do not oppress them with your tongue, or hands, or in any other way. Always work for the good of mankind. Never unduly assert yourselves with pride over others, even those who are placed under you. Never use abusive language for anyone, even though he abuses you.” (Noah’s Ark p. 21)
This quote tells Muslims to show mercy towards God’s creatures and not to abuse anyone in any way possible.
Therefore, we should safeguard ourselves from the use of foul language as it stops us from becoming true Muslims.