Concept:
In indirect speech, when the reporting verb is in the past tense (e.g., said), the tense of the reported speech changes according to the rules of sequence of tenses.
For a sentence in the Present Continuous Tense:
\[
\text{is/am/are + verb + ing}
\]
it changes into:
\[
\text{was/were + verb + ing}
\]
in indirect speech.
Step 1: Identify the reporting verb and tense.
Reporting verb = said (past tense)
Reported speech tense = Present Continuous (is singing)
Step 2: Change the tense and remove quotation marks.
“is singing” changes to “was singing”.
The conjunction “that” is used to join the clauses.
Final Answer:
\[
\boxed{\text{Mohan said that the girl was singing a song.}
\]