Q. If a person speaks forbidden words during the daylight in Ramadan, does it invalidate his fast?
A. If we recited the Words of Allah, the Almighty, the All-Powerful:
يَا أَيُّهَا الَّذِينَ آمَنُواْ كُتِبَ عَلَيْكُمُ الصِّيَامُ كَمَا كُتِبَ عَلَى الَّذِينَ مِن قَبْلِكُمْ لَعَلَّكُمْ تَتَّقُونَ
“O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqûn (the pious). 
we will know the wisdom behind the obligation f fasting, which is Taqwa and the worship of Allah, the Most Glorified, the Most High. Taqwa means to abandon forbidden things, and without exception, those things include doing that which is commanded and leaving that which is forbidden. For the Prophet (salAllahu ‘alayhi wa salam) said:
“Whoever does not give up untruthful speech and evil actions, Allah is not in need of him leaving his food and drink.”
According to this, it is confirmed that it is a duty upon the fasting person to avoid unlawful things, including words and deeds. He should not backbite people, nor should he lie, nor should he spread tales among them, nor should he sell forbidden things; he should avoid all unlawful things. If a person undertakes the performance of those things which are commanded, and leaves the unlawful things for a whole month, then his soul will go straight for the remainder of the year.
But unfortunately, many fasting people do not distinguish between the day of their fast and that of not fasting. Instead, they behave in their usual manner, saying unlawful things, such as lying, cheating and other such things. You do not sense the solemnity of fasting from him. These deeds do not invalidate the fast, but they decrease his reward, and it is possible that at the time of accounting, the reward of fasting will be lost.
 Al-Baqarah 2:183
 Reported by Al-Bukhari in the Book of Fasting, in the Chapter: Whoever Does not Give up Untruthful Speech and Acting Upon It (1903).
· Transcribed From: Fatwa Arkan-ul-Islam | Islamic Verdicts on the Pillars of Islam | Volume 2 | Prayer, Zakah, Fasting, Hajj | By: Shaykh Muhammad bin Salih Al-Uthaymeen | Collected & Arranged by: Fahd bin Nasir bin Ibrahim As-Sulaiman | DARUSSALAM