What is the ruling on prolonging the supplication before the Taslim in the last Tashahhud?

There is no harm in prolonging the supplication (du'a) after the Tashahhud. The Prophet, peace and blessings be upon him, used to prolong the standing (qiyam) and sitting (qu'ud) for the Tashahhud, which is known as prolonging the pillars of standing and sitting beyond the duration of other pillars. The Messenger of Allah, peace and blessings be upon him, encouraged supplication at this point, and that the worshipper should choose whatever supplication he likes. Supplication is recommended for both the Imam and others. However, it is preferable for the supplication to be shorter than the Tashahhud and the prayers upon the Prophet, peace and blessings be upon him. The worshipper should balance the actions of the prayer so that they are proportionate in length.