Surah An-Nahl 16:2

He sends down the angels with the Ruh (revelation) of His command to those servants of His whom He wills (saying): "Warn mankind that none has the right to be worshipped but I, so have Taqwa of Me. (2)

Allah sends Whomever He wills with the Message of Tawhid

يُنَزِّلُ الْمَلَائِكَةَ بِالرُّوحِ

(He sends down the angels with the Ruh) refers to the revelation. This is like the Ayat:

وَكَذَٰلِكَ أَوْحَيْنَا إِلَيْكَ رُوحًا مِّنْ أَمْرِنَا ۚ مَا كُنتَ تَدْرِي مَا الْكِتَابُ وَلَا الْإِيمَانُ وَلَٰكِن جَعَلْنَاهُ نُورًا نَّهْدِي بِهِ مَن نَّشَاءُ مِنْ عِبَادِنَا

(And thus We have sent to you a Ruh (revelation) by Our command. You knew not what is the Book, nor what is the faith. But We have made it a light by which We guide whomever We will among Our servants.)[42:52]

عَلَىٰ مَن يَشَاءُ مِنْ عِبَادِهِ

(to those servants of His whom He wills) meaning the Prophets, as Allah says:

اللَّهُ أَعْلَمُ حَيْثُ يَجْعَلُ رِسَالَتَهُ

(Allah best knows where to place His Message.)[6:124]

اللَّهُ يَصْطَفِي مِنَ الْمَلَائِكَةِ رُسُلًا وَمِنَ النَّاسِ

(Allah chooses Messengers from angels and from men.)[22:75]

يُلْقِي الرُّوحَ مِنْ أَمْرِهِ عَلَىٰ مَن يَشَاءُ مِنْ عِبَادِهِ لِيُنذِرَ يَوْمَ التَّلَاقِ - يَوْمَ هُم بَارِزُونَ ۖ لَا يَخْفَىٰ عَلَى اللَّهِ مِنْهُمْ شَيْءٌ ۚ لِّمَنِ الْمُلْكُ الْيَوْمَ ۖ لِلَّهِ الْوَاحِدِ الْقَهَّارِ

(He sends the Ruh (revelation) by His command to whoever among His servants He wills to, that he may warn of the Day of Meeting. The Day when they will (all) come out, nothing about them will be hidden from Allah. Whose is the kingdom this Day? It is Allah's, the One, the Irresistible!)(40:15-16)

أَنْ أَنذِرُوا

((saying): "Warn...") meaning that they should alert them.

أَنَّهُ لَا إِلَٰهَ إِلَّا أَنَا فَاتَّقُونِ

(that none has the right to be worshipped but I, so have Taqwa of Me.) means, 'fear My punishment, if you go against My commands and worship anything other than Me.'