When is it permissible to perform the prostration of gratitude (sujud al-shukr) in a four-rak'ah prayer: is it before or after the tasleem?

Prostration of thankfulness (sujud al-shukr) is not permissible during prayer. According to the Shafi'is and Hanbalis, if a worshipper knowingly performs it, the prayer becomes void, as it constitutes an addition to the prayer. As for the prostration in Surat Sad, scholars differ concerning it: some consider it a prostration of thankfulness (the Shafi'is and Hanbalis), while others consider it a pro prostration of recitation (the Hanafis and Malikis). Consequently, according to the Hanbalis and the more sound opinion among the Shafi'is, the prayer becomes void if the worshipper performs the prostration of Sad, unless he was forgetful or ignorant. However, according to the Hanafis and Malikis, the prayer does not become void by it, and some Shafi'is agree with them on this point.