From the Midrash:
At the exodus from Egypt, the Israelites were able to safely cross the Red Sea and escape the Egyptian army persuing them. Right behind them, the Egyptian soldiers with heavy chariots become stuck in the mud as the waters come rolling back over them. They drown in the sea. Seeing this, the angels break out into song. God sees the angel's rejoicing and reprimands them. God says, "My creatures are drowning in the sea, and you sing songs".
Bereishit Rabbah 78:1
Hadrian (the Roman emperor) asked R. Yehoshua: Do you say that no cadre of angels in heaven praises God twice. Rather, every single day God creates a cadre of new angels, and they sing before Him and then depart?
R. Yehoshua responded: Yes.
Hadrian responded: And where do they go?
R. Yehoshua responded: To the place of their origin.
Hadrian responded: And what is the place of their origin?
R. Yehoshua responded: The River Dinor (lit. river of light/fire).
Hadrian responded: And what is the nature of the River Dinor?
R. Yehoshua responded: It is like the Jordan which never stops, night or day.
Hadrian responded: And where does it come from?
R. Yehoshua responded: From the sweat of the angels as they carry the divine throne.
From Reb Tzadok HaCohen’s discourse on the nature of angels, Sichat Malakhei HaShareit:
“it is possible for the complete person who knows that his purpose in having been created in this world is to do the will of God and for this he was sent, to have the description of “angel” apply to him, truly and consistently. The goal of his heart and his direction are constantly and exclusively inclined to the service of God and the performance of His mission and will, may He be blessed. Just as a person can be called a tailor or a doctor, or something similar, according to his profession which is his exclusive business, so too a human can be called by the description “angel”.