Kindly confirm the authentication of dua Hazrat Dawood alaihe salam. And give the Quaranic reference of this dua.
وَلَقَدْ آتَيْنَا دَاوُودَ مِنَّا فَضْلًا يَا جِبَالُ أَوِّبِي مَعَهُ
وَالطَّيْرَ وَأَلَنَّا لَهُ الْحَدِيدَ
“We bestowed Grace aforetime on David from ourselves: “O ye Mountains!
Sing ye back the Praises of Allah with him! and ye birds (also)! *And
We made the iron soft for him*” (Saba, 34:10)
In this du`a one is asking Allah to soften the heart of the intended
“just as You softened iron for Sayyidina Dawud (as)”.
And you are free to make any du`a you like that contains nothing sinful.