Salim b. 'Abdullah (b. 'Umar) reported on the authority of his father that Allah's Messenger (ﷺ) observed the prayer of a traveller, i. e. two rak'ahs in Mina, and other places; so did Abu Bakr and 'Umar, and 'Uthman too observed two rak'ahs at the beginning of his caliphate, but he then completed four.
Reference: Sahih Muslim 694 a In-book : Book 6, Hadith 21 USC-MSA web (English) : Book 4, Hadith 1477 (deprecated)
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