Someone told me that if one stops to smell the flowers he should remember to make the borei menei b’samim. Where is the source for this in halacha?
Does one make borei menei b’samim if he enjoys the aroma of fresh mowed grass or passes a lawn that is being mowed?
It's in the Kitzur Shulchan Aruch - in סימן נח - דין ברכת הריח.
One may not benefit from a good smell without making a Bracha on it.
Anything one stops to smell needs a Bracha - even entering a flower store to enjoy the smell warrants a Bracha.
However, perfumes used to hide an unpleasant odor, or a perfume warehouse - where they do not expect you to come in and enjoy the smell - do not warrant a Bracha.
The details are then in the other 17 Se'ifim.
Note, however, that the Bracha on flowers is not necessarily בּוֹרֵא מִינֵי בְּשָׂמִים!
If it's a tree, then it's בּוֹרֵא עֲצֵי בְּשָׂמִים.
If it's grass or veggies, then it's בּוֹרֵא עִשְׂבֵי בְשָׂמִים.
The way to differentiate? See the end of Se'if 4
If the stem is at least as hard as flax, and it survives from one year to the next (like rose bushes) then it's a tree - and one says בּוֹרֵא עֲצֵי בְּשָׂמִים.
But if the stems are always soft, one says בּוֹרֵא עִשְׂבֵי בְשָׂמִים.