Articulating the intention for fasting is a bid'ah

Do you have any reports from the Messenger of Allaah (sallallaahu alayhi wa sallam) of articulating the intention of the fast?

No.

Do you have any reports from the Companions (may Allaah be pleased with them) of articulating the intention of the fast?

No.

Did the the Messenger of Allaah (sallallaahu alayhi wa sallam) order anyone to articulate the intention of the fast?

No.

So, where did the Hanaabilah come up with it being mustahabb?

By making Qiyaas that since the intention is articulated in the Talbiyah, it is mustahabb to do so for fasting.

Ibn Taymiyyah said:

بل التلفظ بالنية نقص في العقل والدين

Rather, talaffuZ [saying out with your tongue] your intention is a deficiency in intellect and Diin.