"Do not belittle any good deed.."

عَنْ أَبِي ذَرٍّ قَالَ قَالَ لِيَ النَّبِيُّ صلى الله عليه وسلم لاَ تَحْقِرَنَّ مِنَ الْمَعْرُوفِ شَيْئًا وَلَوْ أَنْ تَلْقَى أَخَاكَ بِوَجْهٍ طَلْقٍ

2626 صحيح مسلم كتاب البر والصلة والآداب باب استحباب طلاقة الوجه عند اللقاء

Abu Dharr reported: The Prophet, peace and blessings be upon him, said, “Do not belittle any good deed, even meeting your brother with a cheerful face.”

Source: Ṣaḥīḥ Muslim 2626

The word Talq in arabic does not just mean cheerful. It's original meaning is for something to be loose, 

So do not belittle any good deed, even meeting your brother with a loose face. The opposite of a loose face is a tense, angry face. So a loose, soft, kind face is even a massive good deed :)