Does forgetting the Tashahhud or doubting its recitation invalidate the prayer, and what should be done?

If a worshipper doubts having omitted the first Tashahhud after completing the prayer, they should disregard that doubt. However, if the doubt occurs during the prayer, according to the Hanbalis, prostration of forgetfulness (sujud al-sahw) is not obligatory. It is recommended by the Shafi'is, obligatory by the Hanafis, and a confirmed sunnah by the Malikis. The more preponderant opinion is that prostration of forgetfulness is recommended for one who doubts the first Tashahhud while still in prayer.