Should the index finger remain raised after completing the Abrahamic prayer in the Tashahhud until the imam says the Taslim, or can it be lowered immediately?

It is sunnah to raise the index finger in the tashahhud from the beginning of "Ashhadu an la ilaha illallah" until the taslim, as an indication of Tawhid (Oneness of God). It was narrated from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings of Allah be upon him) used to raise his finger next to his thumb, "invoking with it." This indicates that the raising begins at the time of supplication in the tashahhud. What has been narrated from the Companions and the Tabi'in is that the gesture with the index finger is intended to signify Tawhid and sincerity. The matter is flexible, so whoever gestures with it has fulfilled the Sunnah.