There is a
Hadeeth which says that the Qurbani animal should be fattened, then it will be
ones vehicle over the siraat bridge.

is the
Hadeeth authentic?

Answer: 

The narration is not authentic. It is recorded in Kashful Khifaa (volume one) that the narration is unknown, hence it should not be quoted.

عظموا ضحاياكم فإنها على الصراط مطاياكم " ، قال إمام الحرمين معناه إنها تكون مركب للمضحين ، وقيل إنها تسهل الجواز على الصراط ، لكن قال ابن الصلاح هذا الحديث غير معروف ولا ثابت فيما علمناه )كشف الخفاء ۳۳۷ )

AND ALLAH TA’ALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST

ANSWERED BY: Mufti Mohammed Desai 

CHECKED AND APPROVED BY: Mufti Muhammed Saeed Motara Saheb D.B 

Date: 10 Safar 1441 

English Date: 09 October 2019