It is reported that ‘Umar – Allah be pleased with him – said:
'Fasting does not mean keeping away from only food and drink, it also means keeping away from lying, falsehood, inanity and swearing [by Allah without need].'
It is reported that Ibrahim Al-Nakha’i – Allah have mercy on him – said:
'They used to say: 'Lying breaks the fast ...''
It is reported that Mujahid – Allah have mercy on him – said:
'There are two practices, if a person can keep himself from them, his fast will be secured for him: back-biting and lying.'
It is related that Abul-’Aliyah – Allah have mercy on him – said:
'The fasting person is in a state of worship as long as he does not backbite.' 
It is reported that Hafsah bint Sirin – Allah have mercy on her – said:
'Fasting is a shield as long as one does not tear it, and tearing it is when you backbite.' 
It is reported that Anas b. Mâlik – Allah be pleased with him – said:
'If the fasting person backbites, his fast is broken.' 
Explaining the meaning of sins breaking the fast, Shaykh Al-Islam Ibn Taymiyah – Allah have mercy on him – states: 
It is related from some of the Salaf that back-biting, tale carrying and the likes break the fast, and it is mentioned as one opinion in the madhab of Imam Ahmad. The final word on this issue is that Allah the Exalted commanded people to fast in order to achieve piety (taqwa), and Allah’s Messenger – praise and peace of Allah be upon him – stated:
'Whoever does not leave off false speech and acting by it; then Allah is not in need of him abandoning his food and drink.'
So if the fasting person does not achieve piety, he has not achieved what is intended through fasting, and so the reward of [his] fast will decrease in accordance [with how much he has gone against the intended goal, i.e. commensurate with his sins].
Righteous deeds have two intended goals: gaining reward and avoiding punishment. If a person fasts while also doing forbidden things, like backbiting, carrying tales between people or consuming what is forbidden and so on, he loses the reward.
Thus, when the imams say [back-biting etc.] does not break the fast, it means that the person who sins is not punished in the way a person who openly breaks his fast would be punished.
And those who said it does break the fast in the sense that the person has not achieved the intended goal behind fasting, or in the sense that he has lost the reward for fasting, then this statement is in agreement with the position of the imams.
One who says it breaks the fast in the sense that the person is to be punished for leaving [the fast], then he is in contradiction to the imams.
A person who disobeys Allah while fasting hasn’t truly grasped the intent behind fasting. The real goal is to achieve piety and obedience of Allah through the abandonment of food, drink and sin. Although a person who backbites, lies or does other sins is not considered to have physically broken his fast, he loses the reward of fasting and in this sense he has broken his fast.
Allah knows best.
 Ibn Abi Al-Shaybah, Al-Musannaf articles 8975, 8980, 8981 and 8982.
 ‘Abd Al-Razzaq Al-San’ani, Al-Musannaf articles 8975.
 Hunad b. Al-Saree, Al-Zuhd article 1204.
 Badr Al-Din Al-Ba’li. Mukhtasar Al-Fatawa Al-Masriyah pp288, 289. 1st edn. 1418H. Dar Al-Kutub Al-’Ilmiyah. Beirut, Lebanon.