Using grep to identify a pattern - linux

Question

I have several documents hosted on a cloud instance. I want to extract all words conforming to a specific pattern into a .txt file. This is the pattern:
ABC123A
ABC123B
ABC765A
and so one. Essentially the words start with a specific character string 'ABC', have a fixed number of numerals, and end with a letter. This is my code:
grep -oh ABC[0-9].*[a-zA-Z]$ > /home/user/abcLetterMatches.txt
When I execute the query, it runs for several hours without generating any output. I have over 1100 documents. However, when I run this query:
grep -r ABC[0-9].*[a-zA-Z]$ > /home/user/abcLetterMatches.txt
the list of files with the strings is generated in a matter for seconds.
What do I need to correct in my query? Also, what is causing the delay?
UPDATE 1
Based on the answers, it's evident that the command is missing the file name on which it needs to be executed. I want to run the code on multiple document files (>1000)
The documents I want searched are in multiple sub-directories within a directory. What is a good way to search through them? Doing
grep -roh ABC[0-9].*[a-zA-Z]$ > /home/user/abcLetterMatches.txt
only returns the file names.
UPDATE 2
If I use the updated code from the answer below:
find . -exec grep -oh "ABC[0-9].*[a-zA-Z]$" >> ~/abcLetterMatches.txt {} \;
I get a no file or directory error
UPDATE 3
The pattern can be anywhere in the line.

Answer 1

You can use this regexp :
~/ grep -E "^ABC[0-9]{3}[A-Z]$" docs > filename
ABC123A
ABC123B
ABC765A

Answer 2

There is no delay, grep is just waiting for the input you didn't give it (and therefore it waits on standard input, by default). You can correct your command by supplying argument with filename:
grep -oh "ABC[0-9].*[a-zA-Z]$" file.txt > /home/user/abcLetterMatches.txt
Source (man grep):
SYNOPSIS
grep [OPTIONS] PATTERN [FILE...]
To perform the same grepping on several files recursively, combine it with find command:
find . -exec grep -oh "ABC[0-9].*[a-zA-Z]$" >> ~/abcLetterMatches.txt {} \;

Answer 3

This does what you ask for:
grep -hr '^ABC[0-9]\{3\}[A-Za-z]$'
-h to not get the filenames.
-r to search recursively. If no directory is given (as above) the current one is used. Otherwise just specify one as the last argument.
Quotes around the pattern to avoid accidental globbing, etc.
^ at the beginning of the pattern to — together with $ at the end — only match whole lines. (Not sure if this was a requirement, but the sample data suggests it.)
\{3\} to specify that there should be three digits.
No .* as that would match a whole lot of other things.