The ruling is that it is unlawful in the case of a man due to the saying of the Prophet (sallAllaahu `alayhi wa sallam):
ما أسفل من الكعبين من الإزار ففي النار
"Whatever is below the ankles of the Izar is in the Fire." (Al-Bukhari no. 5787)
And in his Sahih, Muslim recorded that Abu Tharr, may Allaah be pleased with him, said that Allaah's Messenger (sallAllaahu `alayhi wa sallam) said:
ثلاثة لا يكلمهم الله يوم القيامة ولا ينظر إليهم ولا يزكيهم ولهم عذاب أليم قال فقرأها رسول الله صلى الله عليه وسلم ثلاث مرار قال أبو ذر خابوا وخسروا من هم يا رسول الله قال المسبل والمنان والمنفق سلعته بالحلف الكاذب
"There are three people that Allaah will not speak to on the Day of Judgement, will not look at them, will not purify them; and for them is a painful punishment: one who lets his Izar hang below his ankles, the Mannan with what he gives, and the Munffiq who does his business with false oaths." (Muslim no. 106)
These are two Hadeeths, and their meanings are general, applying to one who drags the garment out of arrogance as well as the one who does it without arrogance. This is because the Prophet (sallAllaahu `alayhi wa sallam) stated it in a general manner without qualification. When the Isbal is done with arrogance then it results in a sin greater and a threat more severe, due to his (sallAllaahu `alayhi wa sallam) saying:
من جر ثوبه من الخيلاء لم ينظر الله إليه يوم القيامة
"Whoever drags his garment out of conceit, Allaah will not look at him on the Day of judgement." (Al-Bukhari no. 5784 and Muslim no. 2085)
It is not possible to think that the prohibition is limited to someone who does it out of arrogance, because the Messenger (sallAllaahu `alayhi wa sallam) didn't qualify the prohibition in the previous two Hadeeth.
He (sallAllaahu `alayhi wa sallam) also did not restrict that in another Hadeeth, where he (sallAllaahu `alayhi wa sallam) said to some of his Companions:
إياك وإسبال الإزار فإنها من المخيلة
"Beware of Isbal, for it is from conceit." (Abu Dawud no. 4084)
So he made all cases of Isbal arrogance, because it is normally not done without that. As for one who does not do it for that reason, then it is a means that leads to that. And a "means" holds the same ruling as an "end."
Furthermore, this is a form of waste, and letting one's garment drag on the ground makes that garment susceptible to impurities on the ground. This is why, when 'Umar, may Allaah be pleased with him, saw a young man letting his garment touch the ground, he said, "Raise your garment, for it is more pious to your Lord and cleaner for your garment."
As for the case when Abu Bakr As-Siddiq, may Allaah be pleased with him, said: "O Messenger of Allaah! "My Izar falls unless I take care (to hold it up)." The Prophet (sallAllaahu `alayhi wa sallam) said to him:
لست ممن يصنعه خيلاء
"You are not one of those who do so out of conceit." (Al-Bukhari no. 5784 and Abu Dawud no. 4085)
So his (sallAllaahu `alayhi wa sallam) point is that whoever tries to raise his garment when it is loose, then he is not dragging his garment out of arrogance, he merely cannot prevent it. It is just because it is hanging down while he takes care to raise it. So there is no doubt that this is a valid excuse. As for one who purposefully lets it hang, be it a Bushta, pants, Izar, shirt, then this falls under the threat, and he is not excused from dragging his garment. This is because the authentic Hadeeths that prohibit Isbal are general in wording, meaning, and objective. So it is obligatory for every Muslim (male) to beware of Isbal and to have Taqwa of Allaah in this matter, not allowing his garments to hang below his ankles in accordance with the authentic Hadeeths that warn him of Allaah's anger and punishment.
And with Allaah is the facilitation to do what is right.