SB 1.3.11

ṣaṣṭham atrer apatyatvaṁ
 vṛtaḥ prāpto ’nasūyayā
ānvīkṣikīm alarkāya
 prahlādādibhya ūcivān
Synonyms: 
ṣaṣṭham — the sixth one; atreḥ — of Atri; apatyatvam — sonship; vṛtaḥ — being prayed for; prāptaḥ — obtained; anasūyayā — by Anasūyā; ānvīkṣikīm — on the subject of transcendence; alarkāya — unto Alarka; prahlāda-ādibhyaḥ — unto Prahlāda and others; ūcivān — spoke.
Translation: 
The sixth incarnation of the puruṣa was the son of the sage Atri. He was born from the womb of Anasūyā, who prayed for an incarnation. He spoke on the subject of transcendence to Alarka, Prahlāda and others [Yadu, Haihaya, etc.].
Purport: 

The Lord incarnated Himself as Dattātreya, the son of Ṛṣi Atri and Anasūyā. The history of the birth of Dattātreya as an incarnation of the Lord is mentioned in the Brahmāṇḍa Purāṇa in connection with the story of the devoted wife. It is said there that Anasūyā, the wife of Ṛṣi Atri, prayed before the lords Brahmā, Viṣṇu and Śiva as follows: “My lords, if you are pleased with me, and if you desire me to ask from you some sort of blessings, then I pray that you combine together to become my son.” This was accepted by the lords, and as Dattātreya the Lord expounded the philosophy of the spirit soul and especially instructed Alarka, Prahlāda, Yadu, Haihaya, etc.