Thawbān (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "For each forgetfulness (in prayer) there are two prostrations after making Taslīm."
What is in meant in the Hadīth is that any forgetfulness that occurs during the prayer, whether by increasing, decreasing, or doubting something; it necessitates prostration of forgetfulness. This Hadīth is a proof for those who view that the prostration of forgetfulness is after the Salām. Reconciling the evidences in this regard necessitates restricting the prostration that comes after the Salām to two cases: if there is a decrease in the prayer, or if one is in doubt and bases his decision upon a preponderant assumption; apart from these two cases, the prostration should be made before the Salām