﴿ إِذْ دَخَلُوا۟ عَلَيْهِ فَقَالُوا۟ سَلَٰمًۭا ۖ قَالَ سَلَٰمٌۭ قَوْمٌۭ مُّنكَرُونَ ﴾

“When those [heavenly messengers] came unto him and bade him peace, he answered, “[And upon you be] peace!” - [saying to himself,] “They are strangers.””

Please check ayah 51:30 for complete tafsir.