The Quran

إِذ يوحي رَبُّكَ إِلَى المَلائِكَةِ أَنّي مَعَكُم فَثَبِّتُوا الَّذينَ آمَنوا ۚ سَأُلقي في قُلوبِ الَّذينَ كَفَرُوا الرُّعبَ فَاضرِبوا فَوقَ الأَعناقِ وَاضرِبوا مِنهُم كُلَّ بَنانٍ

Muhammad Asad

Lo! Thy Sustainer inspired the angels [to convey this His message to the believers]: "I am with you !"¹ [And He commanded the angels:] "And give firmness unto those who have attained to faith [with these words from Me]:² 'I shall cast terror into the hearts of those who are bent on denying the truth; strike, then, their necks, [O believers,] and strike off every one of their finger-tips!"³

The phrase "I am with you" is addressed (through the angels) to the believers - "for, the purport of these words was the removal of fear, since it was the Muslims, and not the angels, who feared the deniers of the truth" (Razi).
The following is, again, addressed to the believers (Razi). Verse 10 of this surah makes it clear that the aid of the angels was purely spiritual in nature; and there is no evidence anywhere in the Qur'an that they did, or were meant to, participate in the battle in a physical sense. In his commentary on the above verse, Razi stresses this point repeatedly; among modern commentators, Rashid Rida emphatically rejects the legendary notion that angels actually fought in this or any other of the Prophet's battles (see Manar IX, 612 ff.). It is mainly on the basis of Razi's interpretation of this passage that I have interpolated, in several places, explanatory clauses between brackets.
I.e., "destroy them utterly".