O you who have believed, why do you say that which you do not do? Most hateful it is in the sight of Allah that you should say that which you do not do.
(Al-Qur’an – 61:2-3)
There should be complete agreement between a true Muslim’s word and deed: he should carry into effect whatever he says, and when he has no intention of doing it, or has no power for it, he should not say it. To say one thing and do another is one of the most hideous characteristics of man, in the sight of Allah. The Holy Messenger (peace and blessings of Allah be with him) has explained that a person’s being characterised by this quality is a sign that he is not a believer but a hypocrite.
According to a hadith, he said, "The hypocrite has three signs even if he offered the Prayer and observed the Fast, and professed to be a Muslim: That when he spoke he lied; when he made a promise, he broke it; and when he was entrusted with something, he proved dishonest." (Bukhari, Muslim)
In another hadith he said: "Four characteristics are such that the one who has all the four, is a hypocrite through and through, and the one who has one of these, has one characteristic of hypocrisy in him until he gives it up: that when he is entrusted with something, he proves dishonest; when he speaks, he lies; when he makes a promise, he breaks it; and when he quarrels he crosses all limits of morality and decency." (Bukhari, Muslim)
The jurists of Islam have almost unanimously held the view that if a person makes a pledge to Allah (e.g. vows to do some thing), or enters an agreement with others, or promised somebody to do something, it is obligatory for him to fulfil it, unless the thing he has promised is by itself sinful. If it is sinful, he should not observe or carry out the agreement or promise, but should expiate its violation as mentioned in Surah Al-Ma’idah: 89.
The object is to reprove those people who talked much and made tall promises to fight and lay down their lives in the cause of Islam, but when the time came of their test and trial, they fled the battlefield.