In a Hadith related from the Messenger of Allah, may Allah bless him and grant him peace, we read where he says, "From the exellence of a man's Islam is leaving what does not concern him"...ie Leaving that which is not important or benefitting him, whether in speech, action or thought.
Thus, 'the excellence of a man's Islam' is its perfection, such that one remains steadfast in the submission to the Commands and prohibitions of Allah and surrenders to Hisrulings in accordance to His destiny and decree. The reality of 'that which does not concern him' is that which is not needed for a worldly or next-worldly necessity, and does not aid in attaining his Lord's good pleasure, such that it is possible to live without it....such as excessive acts and unnecessary speech.
This Hadith may well be taken from Allah Most High's saying:--"And who shun all vain things"(Qur'an 23:3).
For further reading, see Mulla Ali al-Qari's
"Commentary On Mishkat al-Masabih".