die Quelle
source
A place where information can be found, e.g. a book, an article, or a person.
Für seine Abschlussarbeit musste er viele wissenschaftliche Quellen zitieren.
For his thesis, he had to cite many academic sources.

source
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source
A place where information can be found, e.g. a book, an article, or a person.
Für seine Abschlussarbeit musste er viele wissenschaftliche Quellen zitieren.
For his thesis, he had to cite many academic sources.

All meanings of 'Quelle' (source) share a common idea: it is the origin or beginning of something. That can be the beginning of a river, a piece of information, or a problem.
Wir müssen die Quelle des Problems finden.
We have to find the source of the problem.
You can easily recognize the exact meaning of 'Quelle' (source) from context. Words like 'Wasser' (water) or 'Berg' (mountain) point to the origin of a river, while 'Buch' (book) or 'Artikel' (article) refer to a source of information.
Das Wasser aus dieser Bergquelle ist sehr kalt.
The water from this mountain spring is very cold.
In academic or journalistic contexts, a source is often 'angegeben' (given/cited), 'zitiert' (quoted/cited), or 'geprüft' (checked). A good source is 'zuverlässig' (reliable) or 'seriös' (reputable).
Der Journalist nannte eine anonyme Quelle aus dem Ministerium.
The journalist named an anonymous source from the ministry.
When 'Quelle' (source) describes the cause of a feeling or state, the preposition 'für' (for) is often used. One thing is then the source of something.
Seine Kinder sind für ihn eine unerschöpfliche Quelle der Freude.
His children are an inexhaustible source of joy for him.
In computer science, 'Quelle' (source) or 'Quellcode' (source code) is used as a technical term for the readable text of a computer program. This meaning is uncommon outside the IT industry.
Der Entwickler muss den Fehler im Quelltext beheben.
The developer has to fix the error in the source code.
reason / ground
'der Grund' and 'die Quelle' can both point to the origin of something. 'der Grund' usually names why something happens or why someone does something. 'die Quelle' is more about where something comes from or what causes something to arise. With problems, feelings, or information, this can seem similar. You can confuse them when talking about the cause of something. More natural is: 'Grund' for a decision or behavior, and 'Quelle' for the origin of something, for example stress, noise, or information.

Die Ärztin suchte nach der Quelle der starken Schmerzen.
The doctor looked for the source of the severe pain.
Der Grund für ihre Verspätung war ein langer Stau auf der Autobahn.
The reason for her delay was a long traffic jam on the highway.