One day, I promised my Lord that I would perform the fajr (dawn) prayer on time once I recovered from sickness. I actually recovered and regularly woke up to perform the fajr prayer on time. With the course of time, however, I slackened to perform the fajr prayer on time. I breached the promise that I had made to Allah Almighty. What should I do so that Allah will forgive my sin and that I can perform the prayer on time again? Does negligence in performing the fajr prayer on time constitute a sin that disallows one to lead the people in prayer? Please advise.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Performing the Fajr prayer on time is an obligation in principle. Vowing to perform it or promising Allaah to do so just asserts the obligation. When a person says, "If Allaah heals me, I will do such-and-such," then this is considered a vow, not merely a promise. The difference is that, unlike in the case of a promise, failing to fulfill a vow requires offering expiation similar to that of a broken oath. In this case, the questioner is required to repent to Allaah Almighty and perform the Fajr prayer on time and in congregation at the masjid, if he is a man. At the same time, he is required to offer expiation for not fulfilling the vow. He should also seek the help of a relative, a friend, an alarm clock, and so on in order to wake up to perform this prayer on time. Failing to wake up for performing the Fajr prayer on time without a Sharee'ah-approved justification is a sin that disallows one to lead people in prayer because it constitutes apparent disobedience if it is insistently committed. 

Allaah Knows best.