The first to pray was Ali.
أول من صلى علي.
No references are available.
Tirmidhi (3734) and Tayalisi (2875), and from him Ahmad (3542), all narrated it through the hadith of Abu Balaj, from Amr bin Maymun, from Ibn Abbas, who related it. The wording given is that of Tirmidhi. The wording of Abu Dawud Tayalisi is, "The first to pray with the Prophet ﷺ after Khadijah was Ali." And on another occasion he said, "to accept Islam." Abu Balaj's name is Yahya bin Abu Salim.Tirmidhi said, "This is a gharib hadith from this angle."I say, its chain is hasan on account of Abu Balaj, for there is disagreement about him, though his hadith is hasan as long as its text contains no objectionable element, and this is not something he alone transmitted.Tirmidhi said, "The scholars have differed on this matter. Some said the first to accept Islam was Abu Bakr Siddiq, and others said it was Ali. Some scholars said the first among the men to accept Islam was Abu Bakr, while Ali accepted Islam when he was a boy of eight years, and the first among the women to accept Islam was Khadijah."