Chapter 1 - Some Basic Concepts of Arithmetic and Algebra - 1.2 - Prime and Composite Numbers - Problem Set 1.2 - Page 13: 44

$123=3\times41$.

Since the sum of the digits of 123 is 6, 123 is divisible by 3. When we divide 123 by 3, we get 41. 41is not divisible by any number except 1 and is therefore a prime number. Since 3 and 41 are prime factors, the prime factorization of 123 is $3\times41$.