ENOfficial
Umm Salama said: God’s Messenger taught me to say when the adhan for the sunset prayer was called, “O God, this is the time when Thy night comes on, Thy day retires, and the voices of Thy summoners are heard, so forgive me.” Abu Dawud transmitted it, as did Baihaqi in ad-Da'awat al-kabir.
Source: sunnah.com
Other translations
Arabic
وَعَنْ أُمِّ سَلَمَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: عَلَّمَنِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَن أَقُول عِنْد أَذَان الْمغرب: «اللَّهُمَّ إِن هَذَا إِقْبَالُ لَيْلِكَ وَإِدْبَارُ نَهَارِكَ وَأَصْوَاتُ دُعَاتِكَ فَاغْفِرْ لِي» . رَوَاهُ أَبُو دَاوُدَ وَالْبَيْهَقِيُّ فِي الدَّعَوَاتِ الْكَبِيرِ
sunnah.com