They should say what they usually say when prostrating in Salah in both Sujud-ul-Sahw and Sujud-ul-Tilawah, i.e. "Subhana rabbiyal-A`la (All Glory is due to my Lord, the Most High)." It is obligatory to say this once, but the most complete way is to repeat it three times. It is also desirable to supplicate with the important legislated Du`a' (supplication), as the Prophet (peace be upon him) said:
As for Ruku` (bowing), you should glorify your Lord while performing it, and as for Sujud (prostration), you should strive to supplicate as this is worthy of being answered for you.
The Prophet (peace be upon him) also said:
A servant is nearest to His Lord when they are in Sujud, so supplicate to Allah much.
in his Sahih [authentic] Book of Hadith). The Prophet (peace be upon him) used to say many times in Ruku` and Sujud:
All Glory is due to You, O Allah, our Lord, and all Praise is due to You, O Allah, forgive me.
(Agreed upon by Al-Bukhari and Muslim as reported on the authority of
`Aishah - may Allah be pleased with her). He (peace be upon him) also used to say in Ruku` and Sujud:
All Glorious, All Holy, Lord of the Angels and the Spirit.
(Related by Muslim in his Sahih)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.