Answered by Ustadh Salman Younas
Question: Is the forgetfulness prostration necessary when a person is doing at-tahiyat in the second rak’ah and then begins to read durood shareef but stops and rises before it is finished?
Answer: assalamu `alaykum
The forgetfulness prostration would be necessary for one who recited blessings upon the Prophet (Allah bless him and grant him peace) after the tashahhud in the first sitting of a four cycle prayer if the recitation was the extent of “allahuma salli `ala muhammad wa ` ala aali muhammad.”
The same would be the case if one simply sat after completing the tashahhud in the first sitting of a four cycle prayer for a period wherein the above could have been said. [Ibn `Abidin, Radd al-Muhtar]
This is because standing up after the first sitting without undue delay is considered necessary (wajib), and leaving something that is necessary (wajib) forgetfully requires the forgetfulness prostration in the Hanafi school. Thus, if one did the above action intentionally, it would be necessary for him to repeat his prayer within that prayer time, and would remain recommended to do so after its exiting.