Sunan Abi Dawud Book 2, Hadith 378
When he stood up, he said the takbir (Allah is most great) and said. . . .
I asked ‘A’ishah: By what words the Prophet(ﷺ) used to begin his prayer when he stood up at night (to offer tahajjud prayer). She said: When he stood up at night, he began his prayer by saying: O Allah, Lord of Jibra’il, Lord of Mik’ail, and Lord of Israfil, Creator of the Heavens and the Earth, the Knower of what is seen and of what is unseen; Thou decides between Thy servants in which they used to differ. Guide me to the truth where there is a difference of opinion by Thy permission. Thou guidest anyone Thou wishes to the right path.
There is no harm in uttering supplication in prayer, in its beginning, in its middle, and in the end, in obligatory prayer or other.