Vitthala means the one who stands on veeta(Brick), is an avatar of Shriman Naryana aka Panduranga(whose colour is like that of mud pandu).
Vitthala is worshipped by Vaishnavas all over the world and he is most popular amongst Dwaitins and Varkari sect people.
Where is Vitthala avatar mentioned in Scriptures or in other words which purana(s) mention sthala(place) mahatmya of Pandharpur.
PS: Quote the story for taking avatar if possible.
Rise of a Folk Deity: Vitthal of Pandharpur by R.C Dhere says that the Panduranga Mahatmya is a text attributed to Skanda Purana, Padma Purana and Vishnu Purana.
There are three extant Puranic style Sanskrit texts about Pandharpur, all entitled “Panduranga Mahatmya.”
One of them claims to be part of the Skanda Purana another claims to be part of the Padma Purana and the third, the only known copy of which is in my own collection, claims to belong to the Visnu Purana.
The first two of these texts are exceptionally important for the study of Pandharpur and Vitthal.
Unfortunately I wasn't able to find any of the texts in any of the Puranas so it is possible they maybe lost or are mainly in Sanskrit and haven't been translated.
This Wikipedia article gives an account of various Panduranga Mahatmyas, from the Varkari sect, the the Sanskrit Brahmins and the Marathi Brahmins, of which the Panduranga Mahatmya by Sridhar is famous.
Pg 22 of the book gives an account of names attributed to Pandharpur.
Place names like "Pandurangaksetra ,” "Pundrikaksetra," and “Paundarikaksetra” occur in both the Panduranga Mahatmya from the Skanda Purana and the one from the Padma Purana. The Mahatmya from the Padma Purana (chapters 1 and 2) also uses the names “Dindiravana," “Lohadanda Ksetra,” “Laksmitirtha,” and "Mallikarjunavana.“
Pg 31 gives Four reasons for Panduranga's manifestation.
FOUR REASONS FOR THE MANIFESTATION
In addition to the story of Krsna’s appearance to Pundalik, the Panduranga Mahatmyas from the Skanda and Padma Puranas cite three other reasons why Lord Vitthal manifested himself.
Indeed, what these Mahatmyas' story of Vitthal consists of is the narrative of his appearance in Pandharpur for these four reasons. The reasons are as follows:
Pleased with the way that Pundarik(Or Pundalik) served his mother and father, Krsna carne to his asram in the form of a cowherd in order to give him a boon or a chance to have his darsan (PMSP, chapters 1 and 7) This is the reason I have discussed so far.
Because Krsna allowed Radha to be close to him even in Dvaraka, Rukmini got angry with him and came sulking to the Dindira Forest (Dindiravan) on the bank of the Bhima. In order to assuage her anger, Krsna carne first to Gopalpur along with the cows and cowherds. Then, leaving his whole retinue there, he went alone in the garb of a cowherd to see Rukmini (PMPP, chapter 1).
In order to kill a demon named Dindirava, who had become swollen with arrogance in the Dindirava (or Dindira) forest, Visnu took the form of Mallikarjun Siva. He killed the demon with an iron rod (PMPP, chapter 2).
When a beautiful young woman named Padma had undertaken asceticism in order to get a good husband, God took a form even more beautiful than hers and manifested himself before her. She became infatuated when she saw the god's beauty; her clothes fell away and her hair came loose. By the god’s boon, a tirtha came into being. It was named Muktakesi (“Loose- haired”) in honor of her (PMSP, 3.4of.; 12.32; PMPP, 2114).
Additionally this JSTOR article (you may have to register to read the article I am not sure if I am allowed to directly quote it) on The "Pāṇḍuranga-Māhātmya" of Śrīdhar gives an account of Indra being cursed by Vrtasura to become a brick and Vishnu takes avatar as Panduranga to free Indra by stepping on the brick.
