What is the ruling on someone who sits between the two rak'ahs of Fajr to recite the Tashahhud and then stands up, and is he required to perform the prostration of forgetfulness (sujud al-sahw)?

The prayer of the worshipper is valid, and he is not required to perform the prostration of forgetfulness (sujud sahwi) for this slight addition. A brief, inadvertent sitting during the witr prayer does not necessitate prostration according to the Malikis unless it is equivalent to the duration of the tashahhud. Assuming the prostration of forgetfulness were prescribed, it would be performed after the salutations (taslim), and the prayer would not be invalidated by intentionally omitting it.