ANSWER:

The prophet salla Allahu alaihi wa sallam only offered 2 rakah of thuhur thinking that he offered 4! When this was brought to his attention, he came down and offered the remaining 2 and then offered forgetfulness prostrations because he added salam after the 2nd rakah which is not from the prayer. 

https://www.assimalhakeem.net/2015/12/02/sujood-as-sahu-prostrations-of-forgetfulness/

Shiekh in Sunan Abu dawood 1014 , it says that the prophet(PBUH) offered 2 rakahs of prayer, and when a person asked him “has the prayer been shortened?”, he offered 2 rakahs and made 2 prostrations at the end. Iam confused, what does this hadith mean?why did the prophet( (PBUH) offer 2 prostrations at the end? did he repeat the prayer?