Step 1:
The mantra promotes unity, harmony, and collective thinking.
Step 2:
This famous verse occurs in the Rigveda.
Step 3:
The mantra encourages people to:
\[
\text{Walk together, speak together, and think together.}
\]
Step 4:
Hence, the mantra belongs to:
\[
\boxed{\text{(1) Rigveda}}
\]