The Messenger of Allah ﷺ taught me to say when the adhan for the sunset prayer was called; "O Allah, this is the time when Thy night comes on, Thy day retires, and the voices of Thy summoners are heard, so forgive me."  

عَلَّمَنِي رَسُولُ اللَّهِ ﷺ أَنْ أَقُولَ عِنْدَ أَذَانِ الْمَغْرِبِ اللَّهُمَّ إِنَّ هَذَا إِقْبَالُ لَيْلِكَ وَإِدْبَارُ نَهَارِكَ وَأَصْوَاتُ دُعَاتِكَ فَاغْفِرْ لِي