it's not only sunnah..it's obligatory
''Al-mosbel'' means the one whose clothes under his ankles are cursed...this is one of the dress codes of Muslim men which they totally ignore..
Al-Bukhari reported that the Prophet (peace and blessings of Allah be upon him) said: "Whatever of the izaar (lower garment) is below the ankles is in the Fire." (al-Bukhari, no. 5787)
The Prophet (peace and blessings of Allah be upon him) said: "There are three whom Allah will not look at or praise on the Day of Judgment and theirs will be a painful punishment: the one who wears his garment below his ankles, the one who reminds others of his favors, and the one who sells his product by means of making false oaths" (reported by Muslim, no. 106)
The Prophet (peace and blessings of Allah be upon him) said: "Isbaal (wearing one’s garment below the ankles) may apply to the izaar (lower garment), the shirt or the turban. Whoever allows any part of these to trail on the ground out of arrogance, Allah will not look at him on the Day of Judgment." (reported by Abu Dawud, no. 4085, and al-Nisaa’i, no. 5334, with a saheeh isnaad).