let us see an authentic narration in regards to acting on narrations which speak of mustahab acts

we find in al-kafi,

علي بن إبراهيم، عن أبيه، عن ابن أبي عمير، عن هشام بن سالم، عن أبي عبدالله عليه السلام قال: من سمع شيئا من الثواب على شئ فصنعه، كان له، وإن لم يكن على ما بلغه.

Imam Abu `Abdillah عليه السلام.  He said: One who hears something of reward/thawab upon (doing) something, so he acts on it, it (reward) is for him, even if it was not upon what had reached him.

[al-kafi, vol 2, page 87, narration 1]

regarding authenticity of this narration, we find al-majlisi saying in miraat ul uqool, vol 8, page 112

“hasan but like sahih”

there is a chapter in wasail-us-shia (1/80-82)

english translation for the chapter can be seen here