Once you are in the position of prostration, you should try to be at ease, taking your time, and concentrating on Allah. This position is one in which a Muslim is closest to Allah, so supplication should be made.
While in the position of prostration, it is compulsory to say at least once, and it is recommended to say three or more times:
Transliteration: subḥaana rabbiyal-a'laa
Translation: How far from imperfections my Lord is, The Most High.
It is important to note that in this position of prostration, it is not allowed to recite any Qur'an, because of what was narrated by 'Ali ibn Abī Ṭālib (may Allah be pleased with him) that:
"The Messenger of Allah (may the peace and blessings of Allah be upon him) forbade me from reciting (the Qur'an) in a state of bowing and prostration." [Muslim: 480]
As well as the narration of Ibn 'Abbās (may Allah be pleased with him) that the Prophet (may the peace and blessings of Allah be upon him) said:
"I have indeed been forbidden from reciting the Qur'an in rukoo' or sujood. Therefore, in the rukoo', glorify the supremity of your Lord, Mighty and Sublime, and as for the sujood, exert yourselves in supplication, for it is most likely that you will be answered." [an-Nasā'ī: 1046]
Instead of reciting Qur'an in the prostration we are encouraged to supplicate to Allah, because of the following hadith:
"The Messenger of Allah (may the peace and blessings of Allah be upon him) said: "The nearest the slave [of Allah] comes to his Lord is when he is prostrating, so make supplication (in this state)." [Muslim: 482]
From the above hadith we realise the importance of the prostration in prayer, it is a moment in which we are the closest to Allah. This means that we should pay utmost attention to performing the prostration correctly and not rushing it, the same also applies to bowing in prayer, as described in the following narration:
Abū Qatādah (may Allah be pleased with him) reported that the Messenger of Allah (may the peace and blessings of Allah be upon him) said:
"The worst thief is one who steals in his prayer." The companions asked, "How can someone steal from his prayer?" The Prophet answered, "He does not complete his bowing and prostration with perfection." Or he said, "He does not make his back straight in bowing and prostration." [Ahmad: 22642]