Kindness is found not just in actions and in words, but also in inaction and silence. We may not often think about that because it's hard to notice. After all, if someone bites his tongue and doesn't make a cutting remark, we will never know we could have been hurt but were spared by his kindness. But the Sufi rule of speech is a good one — if we all used kindness as a "speech barrier", much less negativity would be verbalized.
— Will Glennon, Practice Random Acts of Kindness by Editors of Random Acts of Kindness