Mishkat al-Masabih #622

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.