O Allah, I ask You for a sound heart and a truthful tongue. — Daily Supplications
اللَّهُمَّ إِنِّي أَسْأَلُكَ قَلْبًا سَلِيمًا وَلِسَانًا صَادِقًا
Transliteration: Allaahumma innee as'aluka qalban saleeman wa lisaanan saadiqaa
Translation: O Allah, I ask You for a sound heart and a truthful tongue.
Reference: Ahmad, Tirmidhi