Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 14: The Aśvamedha Sacrifice
Text 1.14.58

ततः प्रीतमना राजा प्राप्य यज्ञमनुत्तमम्।
पापापहं स्वर्नयनं दुष्करं पार्थिवर्षभैः॥

tataḥ prīta-manāḥ rājā = King Daśaratha’s mind became pleased; prāpya = having performed; yajñam = sacrifice; anuttamam = the supreme; pāpa-apaham = that destroys sins; svar-nayanam = and leads one to heaven; duṣkaram = and which is difficult to be executed; pārthiva-rṣabhaiḥ = by other emperors.

Having performed the supreme sacrifice that destroys sins and leads one to heaven and which is difficult to be executed by other emperors, King Daśaratha’s mind became pleased.

The supreme sacrifice here is the Aśvamedha which destroys sinful reactions that obstruct one from having sons. By granting the sacrificer sons, it leads him to the heavenly planets.