All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Prostration during prayer is regarded as one of the main places in which the supplication is answered, so one should be mindful thereof and be diligent in supplicating therein. It is related in Saheeh Muslim that the Prophet, sallallaahu ‘alayhi wa sallam, said, "The closest the slave can be to his Lord is in prostration," and: "As for bowing, then glorify your Lord therein; and as for prostration, then be diligent in supplicating, as you will likely be answered."
Elongating the prostration is permissible in general. However, specifying the final prostration or other than it is something for which there is no evidence. So if it happened once or by coincidence, then there is nothing wrong with that, but one should not persist in it. This is in reference to someone praying by himself. Someone leading the prayer should not elongate it so as not to cause hardship to those behind him. It was narrated by Muslim in his Saheeh that the Prophet, sallallaahu ‘alayhi wa sallam, said, "If anyone of you leads the people in prayer, he must make it light, for verily among them are the young, the old, the sick, and the weak. But if he prays alone, then he can pray however he wishes."
An-Nawawi commented on this Hadeeth by saying:
The command is given to the Imam to lighten the prayer without neglecting its Sunan and its direct objectives. However, if he prays by himself, then he can elongate it however long he likes in the parts that can be elongated, which are the standing, the bowing, the prostration, the Tashahhud, and the sitting between the two prostrations.
Allah knows best.