The sounder view regarding this issue is as follows:

As for a voluntary prayer—especially the night prayer—it is sunnah for the worshipper to seek refuge with Allah when he comes to a verse of warning and to ask when he comes to a verse concerning mercy, following the example of the Messenger of Allah, peace and blessings be upon him. This is more conducive to attentiveness of the heart and more effective in reflecting deeply; moreover, the night prayer is a prayer in which it is sunnah to lengthen the recitation, bowing, prostration, and the like.

As for an obligatory prayer, however, it is not sunnah, even though it is permissible.

If someone were to say, “What is your evidence for making this distinction, when you maintain that whatever is established in a voluntary prayer is likewise established in an obligatory prayer? Then let it be sunnah in an obligatory prayer just as it is in a voluntary prayer.”

The answer is that the Messenger, peace and blessings be upon him, performed three prayers every day and night in which the recitation was audible. He would recite verses containing warnings and verses concerning mercy, yet the Companions who transmitted the description of the Messenger’s prayer did not report that he did this in the obligatory prayer. Had it been sunnah, he would have done it; and had he done it, it would have been transmitted. Since it was not transmitted, we know that he did not do it; and since he did not do it, we know that it is not sunnah.

The Companions, may Allah be pleased with them, were keen to observe every movement and stillness of the Prophet, peace and blessings be upon him. They would even infer his recitation in prayers recited silently from the movement of his beard. (1) When he paused between the opening takbir and the recitation, Abu Hurayrah asked him what he said. (2) Had he paused at a verse of warning in order to seek refuge, or at a verse concerning mercy in order to ask, they would undoubtedly have transmitted that.