"And say, 'My Lord, forgive and have mercy, for You are the best of the merciful.'" It means: You are the most excellent of those who show mercy. God commanded the Prophet, peace be upon him, with this supplication.
﴿وَقُلْ رَبِّ اغْفِرْ وَارْحَمْ وَأَنْتَ خير الرَّاحِمِينَ﴾ يَعْنِي: وَأَنْتَ أَفْضَلُ مَنْ يَرْحَمُ؛ أَمر الله النَّبِي عليه السلام بَهَذَا الدُّعَاءِ.