The possessive particle 的 is often dropped when I see an expression like this, especially when referring to a family member (such as in this case). Which is more natural in casual conversation and are both correct by grammatical standards?
If you wish to put emphasis on possession, then you should keep 的.
Aw Qirui Guo's Example:
“那是谁的手机” whos phone is that
“我妈妈的” my mother's
Be careful to say 妈妈 and not just 妈 here, especially if the owner of the phone is someone else's mother...haha.
我妈 sounds relative more mature and probably more common
我妈妈 sounds more cutesy and can sound a bit childish, but more polite
(These are relative comparisons. Their difference is not very big.)
In case where two nouns are understood to have a close personal relationship, or when the first noun or pronoun belongs to a group indicated by the second noun, 的 is often not needed.
(Resources: Chinese textbook)
It will sound casual when you drop 的. And just one thing that I found interesting is that you can say 我媽媽 or 我媽, but you can't say 我的媽 or 我的哥. It needs to be double when you add 的.
Well, please be aware that 妈 and 妈妈 are ofen used by someone self, 妈妈 especially 母亲 is used for other people's mother.
And 妈, 妈妈 and 母亲 is using from natural to formal.
爸爸，你看到我妈在哪里了吗？Hey Daddy, Do you know where is mummy？
汤姆，这是我妈妈. Tom, this is my mum/mother.
杰克，你妈妈身体好些了吗？Jack, do your mother feel better in these days?
杰瑞，请问您母亲叫什么名字？Jerry, may I have your mother's name?