SB 7.4.36

guṇair alam asaṅkhyeyair
 māhātmyaṁ tasya sūcyate
vāsudeve bhagavati
 yasya naisargikī ratiḥ
Word for word: 
guṇaiḥ — with spiritual qualities; alam — what need; asaṅkhyeyaiḥ — which are innumerable; māhātmyam — the greatness; tasya — of him (Prahlāda Mahārāja); sūcyate — is indicated; vāsudeve — to Lord Kṛṣṇa, the son of Vasudeva; bhagavati — the Supreme Personality of Godhead; yasya — of whom; naisargikī — natural; ratiḥ — attachment.
Translation: 
Who could list the innumerable transcendental qualities of Prahlāda Mahārāja? He had unflinching faith in Vāsudeva, Lord Kṛṣṇa [the son of Vasudeva], and unalloyed devotion to Him. His attachment to Lord Kṛṣṇa was natural because of his previous devotional service. Although his good qualities cannot be enumerated, they prove that he was a great soul [mahātmā].
Purport: 

In his prayers to the ten incarnations, Jayadeva Gosvāmī says, keśava dhṛta-narahari-rūpa jaya jagad-īśa hare. Prahlāda Mahārāja was a devotee of Lord Nṛsiṁha, who is Keśava, Kṛṣṇa Himself. Therefore when this verse says vāsudeve bhagavati, one should understand that Prahlāda Mahārāja’s attachment for Nṛsiṁhadeva was attachment for Kṛṣṇa, Vāsudeva, the son of Vasudeva. Prahlāda Mahārāja, therefore, is described as a great mahātmā. As the Lord Himself confirms in Bhagavad-gītā (7.19):

bahūnāṁ janmanām ante
 jñānavān māṁ prapadyate
vāsudevaḥ sarvam iti
 sa mahātmā sudurlabhaḥ

“After many births and deaths, he who is actually in knowledge surrenders unto Me, knowing Me to be the cause of all causes and all that is. Such a great soul is very rare.” A great devotee of Kṛṣṇa, the son of Vasudeva, is a great soul very rarely to be found. Prahlāda Mahārāja’s attachment for Kṛṣṇa will be explained in the next verse. Kṛṣṇa-graha-gṛhītātmā. Prahlāda Mahārāja’s heart was always filled with thoughts of Kṛṣṇa. Therefore Prahlāda Mahārāja is the ideal devotee in Kṛṣṇa consciousness.