I heard a lecture once by a scholar, he said that if one is new to Islam, and does not know what to say during Salat (new to it) then that person can say Tasbeeh in the place of the surahs until they learn them. I would like to know if there are any authentic Ahadith regarding this?
Since a Muslim should pray right away after converting to Islam, it would be reasonable to say that he or she would be unable to memorize the whole text of the prayer in the same day. However, Allah does not put a burden on anyone that they cannot bare.
Therefore the Prophet(P.B.U.H) instructed new Muslims to learn a short supplication
Subhaan-Allaah wal-hamdu Lillaah wa laa ilaaha ill-Allaah wa Allaahu akbar wa laa hawla wa la quwwata illa Billaah
which they can recite in all their 5 daily prayers.
It was narrated that ‘Abd-Allaah ibn Abi Awfa said: A man came to the Prophet (peace and blessings of Allah be upon him) and said: “O Messenger of Allah, teach me something of the Qur’aan that will suffice me, for I cannot read.” He said, “Say: Subhaan-Allaah wa’l-hamdu Lillaah wa laa ilaaha ill-Allaah wa Allaahu akbar wa laa hawla wa la quwwata illa Billaah (Glory be to Allaah, praise be to Allaah, there is no god except Allaah and Allaah is Most great, there is no god except Allaah and there is no power and no strength except with Allaah).” The man made a grabbing gesture with his hand (indicating that he had learned a lot) and said, “This is for my Lord, what is there for me?” He said, “Say: Allaahumma ighfir li warhamni wahdini warzuqni wa ‘aafini (O Allah, forgive me, have mercy on me, guide me, and grant me provision and good health).” He made another grabbing gesture with his other hand and stood up.
(Narrated by al-Nasaa’i, 924; Abu Dawood, 832. Its isnaad was classed as jayyid by al-Mundhiri in al-Targheeb wa’l-Tarheeb, 2/430. al-Haafiz Ibn Hajar indicated that it is hasan in al-Talkhees al-Habeer, 1/236)
He should recite the Quran in his native tounge during his Salat's. At the same time, he should try to memorize short Arabic Suras(like Fatiha and Ikhlas). They are not very hard to learn. If he is unwilling to memorize them, he may simply memorize their translation and recite during the Salat. Islam is an easy religion.