Hadithcore
Mishkat al-Masabih, 955

Shaddad b. Aus said that God's Messenger used to say in his prayer, “O God, I ask You for steadfastness in carrying out what I am commanded and resolution in following right guidance. I ask You to make me grateful for Your favour and to enable me to worship You acceptably. I ask You for a sound heart and a truthful tongue. I ask You for some of the good of what You knowest, I seek refuge in You from the evil of what You knowest, and I ask Your forgiveness for what You knowest.” Nasa’i transmitted it, and Ahmad transmitted something similar.

وَعَن شَدَّادِ بْنِ أَوْسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ فِي صَلَاتِهِ: " اللَّهُمَّ إِنِّي أَسأَلك الثَّبَات فِي الْأَمر والعزيمة عَلَى الرُّشْدِ وَأَسْأَلُكَ شُكْرَ نِعْمَتِكَ وَحُسْنَ عِبَادَتِكَ وَأَسْأَلُكَ قَلْبًا سَلِيمًا وَلِسَانًا صَادِقًا وَأَسْأَلُكَ مِنْ خَيْرِ مَا تَعْلَمُ وَأَعُوذُ بِكَ مِنْ شَرِّ مَا تَعْلَمُ وَأَسْتَغْفِرُكَ لِمَا تَعْلَمُ. رَوَاهُ النَّسَائِيُّ وروى أَحْمد نَحوه

References1 variant
In-Book Reference
Book 4, Hadith 378