Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 39: Sagara’s Sons Search for the Sacrificial Horse
Text 1.39.12

गतिं पुत्रा न पश्यामि रक्षसां पुरुषर्षभाः।
मन्त्रपूतैर्महाभागैरास्थितो हि महाक्रतुः॥

gatiṁ putrā na paśyāmi rakṣasāṁ puruṣarṣabhāḥ
mantra-pūtair
mahā-bhāgair āsthito hi mahā-kratuḥ

gatim = how; putrāḥ = my sons; na = not; paśyāmi = I do see; rakṣasām = the rakṣasas would have entered; puruṣa-ṛṣabhāḥ = O best of men; mantra-pūtaiḥ = purified by Vedic mantras; mahā-bhāgaiḥ = by greatly powerful souls; āsthitaḥ hi = for it is conducted; mahā-kratuḥ = this great sacrifice.

O best of men, my sons, I do not see how the rākṣasas would have entered this sacrifice, for it is conducted by greatly powerful souls purified by Vedic mantras.

Sagara thought that there would not have been any way for the rākṣasas to enter his sacrifice.